<PAGE>   1


                                                                   EXHIBIT 10.18

                            STOCK PURCHASE AGREEMENT

         THIS STOCK PURCHASE AGREEMENT (this "Agreement") is made as of the 26th
day of May, 2000, by and among Rushmore Financial Group, Inc., a Texas
Corporation (the "Company"), and Spectrum Insurance (the "Investor").

                                    RECITALS

         The Company desires to sell, and the Investor desires to purchase, up
to 483,334 shares (collectively, the "Shares") of the Company's Common Stock,
par value $0.01 per share (the "Common Stock"), for the consideration and on the
terms set forth in this Agreement.

         The parties, intending to be legally bound, agree as follows:

         1.       PURCHASE AND SALE OF STOCK

         1.1      SALE AND ISSUANCE OF COMMON STOCK.

                  (a) Subject to the terms and conditions of this Agreement,
         Investor agrees to purchase at the First Closing (as defined in Section
         1.2 hereof), and the Company agrees to sell and issue to Investor at
         the Closing, 166,667 shares of the Company's Common Stock at a purchase
         price of $1.50 per share. The aggregate purchase price shall be paid at
         the First Closing as follows:

                  o   $125,000 by corporate or cashier's check or wire transfer;
                      and

                  o   $125,000 in the form of a non-refundable credit to be used
                      in connection with the agreements (the "Other Agreements")
                      described in b.3. through b.9. of that certain letter from
                      Investor to the Company regarding their future
                      relationship (the "Relationship Letter").

                  (b) Subject to the terms and conditions of this Agreement,
         Investor agrees to purchase on or before 21 days after the First
         Closing, and the Company agrees to sell and issue to Investor at the
         Second Closing (as defined in Section 1.2 hereof), an additional
         166,667 shares of the Company's Common Stock at a purchase price of
         $1.50 per share. The aggregate purchase price shall be paid at the
         Second Closing as follows:

                  o   $125,000 by corporate or cashier's check or wire transfer;
                      and

                  o   $125,000 in the form of a non-refundable credit to be used
                      in connection with the Other Agreements.

                  (c) Subject to the terms and conditions of this Agreement,
         Investor agrees to purchase on or before 45 days after the First
         Closing, and the Company agrees to sell and issue to Investor at the
         Third Closing (as defined in Section 1.2 hereof), an additional 150,000
         shares of the Company's Common Stock at a purchase price of $1.50 per
         share. The aggregate purchase price shall be paid at the Third Closing
         as follows:

                  o   $150,000 by corporate or cashier's check or wire transfer;
                      and

                  o   $75,000 in the form of a non-refundable credit to be used
                      in connection with the Other Agreements.

                  (d) The Company shall (i) use its commercially reasonable
         efforts to prepare and, not later than 60 days following the Third
         Closing Date, to file with the Securities and Exchange Commission (the
         "Commission") a registration statement on an appropriate form under the
         Securities Act of 1933, as amended (the "Securities Act"), relating to
         the offer and sale of the Shares by Investor from time to time in
         accordance with the methods of distribution set forth in such
         registration statement (the "Shelf



<PAGE>   2


         Registration Statement"), and (ii) use its commercially reasonable
         efforts to cause the Shelf Registration Statement to become effective
         under the Securities Act no later than 150 days after the Third Closing
         Date. If the Shelf Registration Statement is not declared effective
         within 150 days after the Third Closing Date, the Company will be
         obligated to issue to Investor 1,000 shares of Common Stock for each
         day in the period beginning on the 151st day after the Third Closing
         Date until the date on which the Shelf Registration Statement is
         declared effective. The Company shall issue such shares to Investor as
         soon as it may practicably do so in compliance with applicable laws and
         in accordance with available exemptions from the registration
         requirements of the Securities Act. Subject to the terms and conditions
         of this Agreement, after the Third Closing Date, if the closing sale
         price of the Company's Common Stock falls below $0.75 per share for
         three consecutive trading days at any time prior to the date on which
         the Shelf Registration Statement is declared effective by the
         Commission, the Company shall issue additional shares of its Common
         Stock to Investor to reduce the effective cost per share of the shares
         of the Common Stock purchased hereunder to that market price. Investor
         understands, acknowledges and agrees that, pursuant to the provisions
         of the Securities Act, Rule 152 promulgated thereunder, and the
         interpretations of the staff of the Commission thereof, the Company
         cannot issue any additional shares of its Common Stock to Investor
         pursuant to this paragraph at any time during the period beginning on
         the date on which the Shelf Registration Statement is filed with the
         Commission and ending after the date on which the offering contemplated
         thereby is completed. If disputes arise under this paragraph (d), the
         parties agree to arbitration by a single arbitrator under the rules of
         the American Arbitration Association. The decision of the arbitrator
         will be final and binding on both parties.

                  (e) Investor agrees to cooperate fully with, and to furnish
         all necessary information to, the Company in connection with the
         preparation and filing of the Shelf Registration Statement.

                  (f) When respect to the convertible preferred described in the
         Relationship Letter, instead of convertible preferred, the Company will
         issue registered Common Stock at the money at the time of issuance,
         except that if the market price at the time of issuance is less than
         $0.75 per share, the market price will be deemed to be $0.75, and if
         the market price is greater than $3.00, the market price will be deemed
         to be $3.00.

                  (g) The Company will cause one representative designated by
         Investor to be elected to the Company's Board of Directors at the next
         meeting of the Company's Board of Directors.

         1.2      CLOSING.

                  (a) The purchase and sale of the Common Stock described in
         Section 1.1(a) above shall take place at the offices of the Company,
         One Galleria Tower, 13355 Noel Road, 6th Floor, Dallas, Texas, on May
         26, 2000, or at such other time and place as the Company and Investor
         shall mutually agree, either orally or in writing (which time and place
         are designated as the "First Closing"). The date on which the First
         Closing is held is referred to herein as the "First Closing Date". The
         purchase and sale of the Common Stock described in Section 1.1(b) above
         shall take place at the offices of the Company, One Galleria Tower,
         13355 Noel Road, 6th Floor, Dallas, Texas, on the last business day of
         the period specified therein, or at such other time and place as the
         Company and Investor shall mutually agree, either orally or in writing
         (which time and place are designated as the "Second Closing"). The date
         on which the Second Closing is held is referred to herein as the
         "Second Closing Date". The purchase and sale of the Common Stock
         described in Section 1.1(c) above shall take place at the offices of
         the Company, One Galleria Tower, 13355 Noel Road, 6th Floor, Dallas,
         Texas, on the last business day of the period specified therein, or at
         such other time and place as the Company and Investor shall mutually
         agree, either orally or in writing (which time and place are designated
         as the "Third Closing"). The date on which the Third Closing is held is
         referred to herein as the "Third Closing Date".

                  (b) At the Closing, the Company shall deliver to Investor a
         certificate representing the shares of Common Stock that Investor is
         purchasing against payment of the purchase price therefor as described
         above.


<PAGE>   3


         2.   REPRESENTATIONS AND WARRANTIES OF THE COMPANY

              The Company hereby represents and warrants to Investor that,
         except as set forth on a Schedule of Exceptions furnished to Investor
         and special counsel for the Investor, specifically identifying the
         relevant Sections(s) hereof, which exceptions shall be deemed to be
         representations and warranties as if made hereunder:

              2.1 ORGANIZATION; GOOD STANDING; QUALIFICATION

              The Company is a corporation duly organized, validly existing, and
         in good standing under the laws of the State of Texas, has all
         requisite corporate power and authority to own and operate its
         properties and assets and to carry on its business as now conducted and
         as presently proposed to be conducted, to execute and deliver this
         Agreement, to issue and sell the Common Stock, and to carry out the
         provisions of this Agreement. The Company is duly qualified and is
         authorized to transact business and is in good standing as a foreign
         corporation in each jurisdiction in which the failure so to qualify
         would have a material adverse effect on its business, properties,
         prospects, or financial condition.

              2.2 AUTHORIZATION.

              All corporate action on the part of the Company, its officers,
         directors and stockholders necessary for the authorization, execution
         and delivery of this Agreement, the performance of all obligations of
         the Company hereunder at the Closing and the authorization, issuance,
         sale, and delivery of the Common Stock being sold hereunder has been
         taken, and this Agreement, when executed and delivered, will constitute
         the valid and legally binding obligation of the Company, enforceable in
         accordance with their respective terms except (i) as limited by the
         applicable bankruptcy, insolvency, reorganization, moratorium, and
         other laws of general application affecting enforcement of creditors'
         rights generally, and (ii) as limited by laws relating to the
         availability of specific performance, injunctive relief, or other
         equitable remedies.

              2.3 VALID ISSUANCE OF COMMON STOCK.

              The Common Stock that is being purchased by the Investor
         hereunder, when issued, sold, and delivered in accordance with the
         terms of this Agreement, for the consideration expressed herein, will
         be duly and validly issued, fully paid, and nonassessable, and will be
         free of restrictions on transfer other than restrictions on transfer
         under this Agreement and under applicable state and federal securities
         laws.

              2.4 SEC FILINGS; FINANCIAL STATEMENTS

                  (a) The Company has filed all forms, reports and documents
              required to be filed by it with the Securities and Exchange
              Commission ("SEC") since January 1, 2000. The Company has
              delivered to the Investor, in the form filed with the SEC, (i) its
              Annual Report on Form 10-KSB for the year ended December 31, 1999,
              (ii) all proxy statements relating to the Company's meetings of
              stockholders (whether annual or special) held since January 1,
              2000, (iii) all other reports or registration statements filed by
              the Company with the SEC (other than Reports on Form 3, 4, or 5
              and Schedules 13G filed on behalf of affiliates of the Company)
              since January 1, 2000, and (iv) all amendments and supplements to
              all such reports filed by the Company with the SEC (collectively,
              the "Company SEC Reports"). The Company SEC Reports (i) were
              prepared in accordance with the requirements of the Securities Act
              or the Securities Exchange Act of 1934, as amended, (the "Exchange
              Act"), as the case may be, and (ii) did not at the time they were
              filed (or if amended or superseded by a filing prior to the date
              of this Agreement, then on the date of such filing) contain any
              untrue statement of a material fact or omit to state a material
              fact required to be stated therein or necessary in order to make
              the statements therein, in the light of the circumstances under
              which they were made, not misleading.

                  (b) Each of the financial statements (including, in each case,
              any related notes thereto) contained in the Company SEC Reports
              (collectively, the "Financial Statements") was prepared in
              accordance with United States Generally Accepted Accounting
              Principles ("GAAP") applied on a consistent basis throughout the
              periods involved (except as may be indicated therein or in the
              notes thereto), and each fairly presents in all material respects
              the consolidated financial position of the Company and its
              subsidiaries as at the respective dates thereof and the
              consolidated results of its operations and cash flows for the
              periods indicated, except that the unaudited interim financial
              statements were or are subject to normal and recurring year-end
              adjustments and such statements do not contain notes thereto.


<PAGE>   4


                  (c) The Company has heretofore furnished to the Investor a
              complete and correct copy of any amendments or modifications,
              which have not yet been filed with the SEC but which are required
              to be filed, to agreements, documents or other instruments which
              previously had been filed by the Company with the SEC pursuant toe
              the Securities Act or the Exchange Act.

         2.5 DISCLOSURE.

         The Company has provided Investor with all the information reasonably
     available to it without undue expense that Investor has requested for
     deciding whether to purchase the Common Stock and all information that the
     Company believes is reasonably necessary to enable such Investor to make
     such decision, including all information available regarding the Company's
     settlement agreement with John Vann. To the best of the Company's
     knowledge after reasonable investigation, neither this Agreement nor any
     other agreements, written statements, or certificates made or delivered in
     connection herewith contains any untrue statement of a material fact or
     omits to state a material fact necessary to make the statements herein or
     therein not misleading.

         3. REPRESENTATIONS AND WARRANTIES OF THE INVESTORS

         Investor hereby represents and warrants to the Company that:

         3.1 AUTHORIZATION.

         Investor has full power and authority to enter into this Agreement, and
     that this Agreement, when executed and delivered, will constitute a valid
     and legally binding obligation of Investor.

         3.2  PURCHASE ENTIRELY FOR OWN ACCOUNT.

         This Agreement is made with Investor in reliance upon Investor's
     representations to the Company, which by Investor's execution of this
     Agreement Investor hereby confirms, that the Common Stock to be purchased
     by Investor and the Common Stock issuable upon conversion thereof
     (collectively, the "Securities") will be acquired for investment for
     Investor's own account, not as a nominee or agent, and not with a view to
     the resale or distribution of any part thereof, and that Investor has no
     present intention of selling, granting any participation in, or otherwise
     distributing the same. By executing this Agreement, Investor further
     represents that Investor does not have any contract, undertaking, agreement
     or arrangement with any person to sell, transfer or grant participations to
     such persons or to any third person, with respect to any of the Securities.

         3.3 RELIANCE UPON INVESTOR' REPRESENTATIONS.

         Investor understands that the Common Stock is not, and any Common Stock
     acquired on conversion thereof at the time of issuance may not be,
     registered under the Securities Act on the ground that the sale provided
     for in this Agreement and the issuance of securities hereunder is exempt
     from registration under the Securities Act pursuant to Section 4(2)
     thereof, and that the Company's reliance on such exemption is predicated on
     the Investor' representations set forth herein.

         3.4 RECEIPT OF INFORMATION.

         Investor has received all the information Investor considers necessary
     or appropriate for deciding whether to purchase the Common Stock. Investor
     further represents that Investor has had an opportunity to ask questions
     and receive answers from the Company and its directors and officers
     regarding the terms and conditions of the offering of the Common Stock and
     the business, properties, prospects, and financial condition of the Company
     and to obtain additional information (to the extent the Company possessed
     such information or could acquire it without unreasonable effort or
     expense) necessary to verify the accuracy of any information furnished to
     Investor or to which Investor had access.

         3.5 INVESTMENT EXPERIENCE.

         Investor represents that Investor is experienced in evaluating and
     investing in private placement transactions of securities of companies in a
     similar stage of development and acknowledges that Investor is able to fend
     for itself, can bear the economic risk of Investor's investment, and has
     such knowledge and experience in financial and business matters that
     Investor is capable of evaluating the merits and risks of the investment in


<PAGE>   5


     the Common Stock. Investor represents that Investor has not been organized
     for the purpose of acquiring the Common Stock.


         3.6 ACCREDITED INVESTOR.

             (a) The term "Accredited Investor" as used herein refers to:

                   (i) A person or entity who is a director or executive officer
             of the Company;

                   (ii) Any bank as defined in Section 3(a)(2) of the Securities
             Act, or any savings and loan association or other institution as
             defined in Section 3(a)(5)(A) of the Securities Act whether acting
             in its individual or fiduciary capacity; any broker or dealer
             registered pursuant to Section 15 of the Securities Exchange Act of
             1934; any insurance company as defined in Section 2(13) of the
             Securities Act; any investment company registered under the
             Investment Company Act of 1940 or a business development company as
             defined in Section 2(a)(48) of that Act; any Small Business
             Investment Company licensed by the U.S. Small Business
             Administration under Section 301(c) or (d) of the Small Business
             Investment Act of 1958; any plan established and maintained by a
             state, its political subdivisions, or any agency or instrumentality
             of a state or its political subdivisions, for the benefit of its
             employees, if such plan has total assets in excess of $5,000,000;
             any employee benefit plan within the meaning of Title I of the
             Employee Retirement Income Security Act of 1974, if the investment
             decision is made by a plan fiduciary, as defined in Section 3(21)
             of such Act, which is either a bank, savings and loan association,
             insurance company, or registered investment advisor, or if the
             employee benefit plan has total assets in excess of $5,000,000 or,
             if a self-directed plan, with investment decisions made solely by
             persons that are accredited investors;

                   (iii) Any private business development company as defined in
             Section 202(a)(22) of the Investment Advisers Act of 1940;

                   (iv) Any organization described in Section 501(c)(3) of the
             Internal Revenue Code, corporation, Massachusetts or similar
             business trust, or partnership, not formed for the specific purpose
             of acquiring the securities offered, with total assets in excess of
             $5,000,000;

                   (v) Any natural person whose individual net worth, or joint
             net worth with that person's spouse, at the time of the purchase
             exceeds $1,000,000;

                   (vi) Any natural person who had an individual income in
             excess of $200,000 in each of the two most recent years or joint
             income with that person's spouse in excess of $300,000 in each of
             those years and has a reasonable expectation of reaching the same
             income level in the current year;

                   (vii) Any trust, with total assets in excess of $5,000,000,
             not formed for the specific purpose of acquiring the securities
             offered, whose purchase is directed by a person who has such
             knowledge and experience in financial and business matters that he
             or she is capable of evaluating the merits and risks of the
             prospective investment; or

                   (viii) Any entity in which all of the equity owners are
             accredited investors.

             As used in this Section, the term "net worth" means the excess of
         total assets over total liabilities. For the purpose of determining a
         person's net worth, the principal residence owned by an individual
         should be valued at fair market value, including the cost of
         improvements, net of current encumbrances. As used in this Section,
         "income" means actual economic income, which may differ from adjusted
         gross income for tax purposes. Accordingly, Investor should consider
         whether Investor should add any or all of the following items to
         Investor's adjusted gross income for income tax purposes in order to
         reflect more accurately Investor's actual economic income: any amounts
         attributable to tax-exempt income received, losses claimed as a limited
         partner in any limited partnership, deductions claimed for depletion,
         contributions to an IRA or Keogh retirement plan, and alimony payments.

             (b) Investor represents and warrants that Investor is an accredited
         Investor.

         3.7 RESTRICTED SECURITIES.

         Investor understands that the Common Stock may not be sold,
     transferred, or otherwise disposed of without registration under the
     Securities Act or an exemption therefrom, and that in the absence of an
     effective registration statement covering the Common Stock or an available
     exemption from registration under the


<PAGE>   6


     Securities Act, the Common Stock must be held indefinitely. In particular,
     Investor is aware that the Common Stock may not be sold pursuant to Rule
     144 promulgated under the Securities Act unless all of the conditions of
     that Rule are met.


         3.8 LEGENDS.

         To the extent applicable, each certificate or other document evidencing
     any of the Common Stock shall be endorsed with the legends substantially in
     the form set forth below:

             The following legend under the Securities Act:

         "THE SHARES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
         SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD, TRANSFERRED,
         ASSIGNED, PLEDGED, OR HYPOTHECATED UNLESS AND UNTIL REGISTERED UNDER
         SUCH ACT, OR UNLESS THE COMPANY HAS RECEIVED AN OPINION OF COUNSEL OR
         OTHER EVIDENCE, SATISFACTORY TO THE COMPANY AND ITS COUNSEL, THAT SUCH
         REGISTRATION IS NOT REQUIRED."

             Any legend imposed or required by the Company's Bylaws or
         applicable state securities laws.

         3.9 FURTHER REPRESENTATION BY FOREIGN INVESTOR.

         If Investor is not a U.S. Person, Investor hereby represents that
     Investor is satisfied as to the full observance of the laws of Investor's
     jurisdiction in connection with any invitation to subscribe for the common
     Stock or any use of this Agreement, including (i) the legal requirements
     with Investor's jurisdiction for the purchase of the Common Stock, (ii) any
     foreign exchange restrictions applicable to such purchase, (iii) any
     governmental or other consents that may need to be obtained, and (iv) the
     income tax and other tax consequences, if any, which may be relevant to the
     purchase, holding, redemption, sale, or transfer of the Common Stock.
     Investor's subscription and payment for, and Investor's continued
     beneficial ownership of, the Common Stock will not violate any applicable
     securities or other laws of Investor's jurisdiction.

         4. MISCELLANEOUS

         4.1 ENTIRE AGREEMENT.

         This Agreement and the documents referred to herein constitute the
     entire agreement among the parties and no party shall be liable or bound to
     any other party in any manner by any warranties, representation, or
     covenants except as specifically set forth herein or therein.

         4.2 SURVIVAL OF WARRANTIES.

         The warranties, representations, and covenants of the Company and the
     Investor contained in or made pursuant to this Agreement shall survive the
     execution and delivery of this Agreement and the Closing.

         4.3 SUCCESSORS AND ASSIGNS.

         Except as otherwise provided herein, the terms and conditions of this
     Agreement shall inure to the benefit of and be binding upon the respective
     successors and permitted assigns of the parties. Nothing in this Agreement,
     express or implied, is intended to confer upon any party other than the
     parties hereto or their respective successors and assigns any rights,
     remedies, obligations, or liabilities under or by reason of this Agreement,
     except as expressly provided in this Agreement.

         4.4 GOVERNING LAW.

             THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED UNDER THE LAWS OF
         THE STATE OF TEXAS WITHOUT REGARD TO CONFLICTS OF LAWS PRINCIPLES.


<PAGE>   7


         4.5 COUNTERPARTS.

         This Agreement may be executed in two or more counterparts, each of
     which shall be deemed an original, but all of which together shall
     constitute one and the same instrument.

         4.6 TITLES AND SUBTITLES.

         The titles and subtitles used in this Agreement are used for
     convenience only and are not to be considered in construing or interpreting
     this Agreement.

         4.7 NOTICES.

         Unless otherwise provided, all notices and other communications
     required or permitted under this Agreement shall be in writing and shall be
     mailed by United States first-class mail, postage prepaid, sent by
     facsimile or delivered personally by hand or by a nationally recognized
     courier addressed to the Company, One Galleria Tower, 13355 Noel Road, 6th
     Floor, Dallas, Texas 75240, facsimile number (972) 450-6001, or to the
     Investor at the address or facsimile number indicated on the signature page
     hereto. All such notices and other written communications shall be
     effective on the date of mailing, confirmed facsimile transfer or delivery.

         4.8 FINDERS FEES.

         Investor agrees to indemnify and to hold harmless the Company from any
     liability for any commission or compensation in the nature of a finders fee
     (and the cost and expenses of defending against such liability or asserted
     liability) for which the Investor or any of its officers, partners,
     employees, or representatives is responsible.

         The Company agrees to indemnify and hold harmless Investor from any
     liability for any commission or compensation in the nature of a finder's
     fee (and the costs and expenses of defending against such liability or
     asserted liability) for which the Company or any of its officers,
     employees, or representatives is responsible.

         4.9 EXPENSES.

         Irrespective of whether the Closing is effected, the Company shall pay
     all costs and expenses that it incurs with respect to the negotiation,
     execution, delivery and performance of this Agreement.

         4.10 ATTORNEYS FEES.

         If any action at law or in equity is necessary to enforce or interpret
     the terms of this Agreement, the prevailing party shall be entitled to
     reasonable attorney's fees, costs, and disbursements in addition to any
     other relief to which such party may be entitled.

         4.11 AMENDMENTS AND WAIVERS.

         Any term of this Agreement may be amended and the observance of any
     term of this Agreement may be waived (either generally or in a particular
     instance and either retroactively or prospectively), only with the written
     consent of the Company and Investor.

         4.12 SEVERABILITY.

         If one or more provisions of this Agreement are held to be
     unenforceable under applicable law, such provision shall be excluded from
     this Agreement and the balance of the Agreement shall be interpreted as if
     such provision were so excluded and shall be enforceable in accordance with
     its terms.

         4.13 ON-GOING PRIVATE PLACEMENT.

         Investor understands, acknowledges and agrees that the Company may
     continue with a private offering as previously described to Investor. If,
     however, the Company were to sell securities in any private offering at a
     price less than $0.75 per share, then, in such event, all purchase prices
     per share pursuant to which Investor is


<PAGE>   8


     purchasing the Shares hereunder and shares referenced in shares in
     Paragraph 1.1.f. will be reduced to such lower private offering price.



         COMPANY SIGNATURE PAGE

         IN WITNESS WHEREOF, the parties have executed this Stock Purchase
Agreement as of the date first above written.



                               RUSHMORE FINANCIAL GROUP, INC.

                               /s/ D. M. (Rusty) Moore, Jr.
                               ----------------------------
                               Chairman, President, and Chief Executive Officer





         INVESTOR SIGNATURE PAGE


                                    INVESTOR:


                                    SPECTRUM INSURANCE

                                             /s/ David Demas
                                             ---------------------------------
                                             President

                                             Address:   2500 West Higgins Road
                                                        Suite 500
                                                        Hoffman Estates, 60195
                                             Fax:       (847) 310-3477