<PAGE>

                                                                   EXHIBIT 10.31
 
                            STOCK PURCHASE AGREEMENT
                            ------------------------

                          Nixon Equipment Corporation

     AGREEMENT made as of January 1, 1997, between DOUGLAS R. COLKITT, M.D. of
2171 Sandy Drive, State College PA 16803 (the "Seller") and EQUIMED, INC., a
Delaware corporation with offices at 2171 Sandy Drive, State College PA 16803
(the "Buyer").

     The parties have reached an understanding with respect to the sale and
purchase of all the outstanding corporate shares of Nixon Equipment Corporation,
a Delaware corporation, engaged in the ownership, management and leasing of real
estate (the "Company").

It is therefore agreed:

     1.1 Sale of Corporate Shares. The Seller shall sell to the Buyer and the
         ------------------------                                            
Buyer shall purchase from the Seller all of the issued and outstanding shares of
stock in the Company for the purchase price described in Section 1.2 below. The
Seller is the owner of all the issued and outstanding stock of the Company as
listed on Schedule 1.1 which is attached hereto and incorporated herein by
          -------------                                                   
reference (the "Shares").

     1.2 Purchase Price. The purchase price for the Shares shall be Four Hundred
         --------------                                                         
Thousand ($400,000.00) Dollars (the "Purchase Price"). The Purchase Price
payable to the Seller shall be reduced by all funds advanced by Buyer to Seller
on December 31, 1996.

     2.1 Closing. The closing of the sale and transfer of the Shares shall take
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place at the location agreed upon by Seller and Buyer (the "Closing"). At the
Closing, Seller shall deliver to the Buyer, free and clear of all encumbrances,
certificates for the Shares which he is required to sell in negotiable form,
with all requisite transfer stamps attached. Upon such delivery, the Buyer shall
deliver to Seller the Purchase Price payable at Closing by a certified or bank
cashier's check or wire transfer.

     3.1 Access, Information and Documents. Seller and Company shall give to
         ---------------------------------                                  
Buyer and to Buyer's counsel, accountants and other representatives full access
during normal business hours to all the properties, books, tax returns,
contracts, commitments, records, officers, personnel and accountants of the
Company and will furnish to Buyer all such documents and copies of documents
(certified to be true copies if requested) and all information with respect to
the affairs of the Company as Buyer may reasonably request. All such information
furnished to Buyer in connection with the transactions contemplated herein shall
be kept confidential, unless the Buyer is compelled to disclose such information
by judicial or administrative process or by other requirements of law, including
but not limited to any applicable securities laws or regulations.
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     3.2 Disclosure Schedules. Seller shall deliver at or prior to Closing, all
         --------------------                                                  
schedules described in this Agreement.

     4. Representations and Warranties. The Seller represents and warrants to
        ------------------------------                                       
Buyer as follows:

     4.1 Organization and Standing of Company. The Company is a corporation duly
         ------------------------------------                                   
organized, validly existing, and in good standing under the laws of Delaware.
Copies of the Company's Certificate of Incorporation, and all amendments thereof
to date, certified by the Secretary of State of Delaware, and of the Company's
Bylaws as amended to date, certified by the Company's Secretary, have been
delivered to the Buyer, and are complete and correct as of the date of this
agreement. The Company is duly licensed or qualified and in good standing as a
foreign corporation in the states listed in Schedule 4.1, which are the only
                                            --------------                  
states where the character of the properties owned by the Company, or the nature
of the business transacted by it, make such license or qualification necessary.

     4.2 Subsidiaries. The Company has no subsidiaries.
         ------------                                  

     4.3 Capitalization. The aggregate number of shares which the Company is
         --------------                                                     
authorized to issue is 1500 common shares, of which 100 shares are issued and
presently outstanding as shown on Schedule 1.1.  All such issued shares have
                                  -------------                           
been validly issued and are fully paid and nonassessable. The Company has no
outstanding subscriptions contracts, options, warrants, or other obligations to
issue, sell, or otherwise dispose of, or to purchase, redeem or otherwise
acquire any of its shares.

     4.4 Share Ownership. Seller represents and warrants that he is the owner,
         ---------------                                                      
free and clear of any encumbrances, of the number of the Company's common shares
set opposite his name on Schedule 1.1.  Seller has full right and authority to
transfer said shares to Buyer, and there are no other shares of the Company
owned or claimed by any other person or entity.

     4.5 Financial Statement. The Seller has delivered to the Buyer copies of
         -------------------                                                 
the following financial statements, all of which are true and complete, to the
best of Seller's knowledge and have been prepared on an accrual basis:

     (a)   Unaudited balance sheets of the Company as of December 31, 1995,
December 31, 1994 and December 31, 1993, together with related unaudited
statements of income and retained earnings and cash flows for the fiscal years
ended on such dates, and the notes thereto;

     (b)   The unaudited balance sheets of the Company as of December 31, 1996,
together with the related unaudited statements of income and retained earnings
and cash flows for the twelve (12) month period ended on such date, and the
notes thereto;

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<PAGE>
 
     (c)   Federal tax returns of the Company as of December 31, 1995, December
31, 1994 and December 31, 1993.

The above financial statements are hereinafter referred to as the "Financial
Statements."

     To the best of Seller's knowledge the Financial Statements: (i) are correct
and complete in accordance with the books and records of the Company; and (ii)
fairly present the financial condition, assets and liabilities of the Company as
of their respective dates and the results of the Company's operations and cash
flows for the periods covered thereby.

     4.6 Absence of Undisclosed Liabilities. Except to the extent listed on
         ----------------------------------                                
Schedule 4.6, the Company has no liabilities of any nature, whether accrued,
absolute, contingent, or otherwise, including, without limitation, tax
liabilities due or to become due, and whether incurred in respect of or measured
by the Company's income for any period prior to December 31, 1996, or arising
out of transactions entered into, or any state of facts existing, prior thereto.
Seller represents and warrants that he does not know or have reasonable grounds
to know of any basis for the assertion against the Company of any liability,
except as listed in Schedule 4.6.

     4.7 Absence of Certain Changes. Since December 31, 1996, to the best of
         --------------------------                                         
Seller's knowledge, there has not been (i) any change in the Company's financial
condition, assets, liabilities, or business, other than changes in the ordinary
course of business, none of which has been materially adverse, (ii) any
declaration, or setting aside, or payment of any dividend or other distribution
in respect of the Company's shares, or any direct or indirect redemption,
purchase, or other acquisition of any of such shares; (iii) any increase in the
compensation payable or to become payable by the Company to any of its officers,
employees, or agents, or any bonus payment or arrangement made to or with any of
them; or (iv) any labor trouble, or any event, damage, loss or condition of any
character, materially and adversely affecting the Company's business or
prospects.

     4.8 Taxes; Tax Audit. The Company has (i) timely filed all tax returns
         ----------------                                                  
required to be filed by it with respect to all taxes payable by the Company
including, but not limited to, income, capital stock, franchise, sales or use,
personal property and real estate taxes ("Taxes"); (ii) timely paid in full all
Taxes shown to have become due pursuant to such tax returns; and (iii) paid all
other Taxes for which a notice of assessment or demand for payment has been
received.  All taxes that the Company is required by the law to pay, withhold or
collect including, but not limited to, payroll taxes and sales and use taxes on
any of the Company's sales or leases of tangible personal property or services,
have been timely paid over to the appropriate tax authority. All taxes of the
Company have been paid or are adequately reserved against on the books of
account of the Company as of December 31, 1996 or reflected on Schedule 4.6 with
respect to any liabilities accruing between December 31, 1996 and Closing. To
the best of Seller's knowledge, the Company has timely filed all information
returns or reports, including Forms 1099, which are required to be filed and has
accurately reported all information required to be included on such returns or
reports.

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<PAGE>
 
     Except as disclosed in Schedule 4.8, the Company's federal income tax
                            --------------                                
returns and state income tax returns have not been audited by the Internal
Revenue Service or any state. The Seller has not received any notice of any tax
audits being conducted by any taxing authority with respect to any tax
liabilities of the Company, including income, sales or other taxes, and Seller
and Company have not received any notice from any taxing authority of an
intention of any taxing authority to conduct any audits.

     4.9 Title to Properties; Mortgages; Liens; Compliance. The Company has good
         -------------------------------------------------                      
and marketable title to all its properties and assets, real and personal, listed
on Schedule 4.9, subject to no security interests, mortgage, pledge, lien,
encumbrance, or charge, except for mortgages and liens shown on Schedule 4.6 as
securing specified liabilities set forth therein (with respect to which no
default exists), and except for minor imperfections of title and encumbrances,
if any, which are not substantial in amount, do not materially detract from the
marketability or the value of the properties subject thereto, or materially
impair the Company's operations, and have arisen only in the ordinary course of
business. All the mortgages, liens, security interests and encumbrances against
the real estate, machinery, equipment and other property owned or leased by the
Company are listed on Schedule 4.6. To the best of Seller's knowledge, all
                      ------------                                        
Company buildings and equipment are in confirmation with all applicable
ordinances and regulations and environmental, building, zoning and other laws
and the real estate is in good operating condition, reasonable wear and tear
accepted.

     4.10 Accounts Receivable. The Accounts Receivable of the Company as of
          -------------------                                              
December 31, 1996 are as shown on Schedule 4.10. Except as disclosed in Schedule
4.10, the Seller is not aware of anything that would indicate that these
Accounts Receivable are not collectible.

     4.11 Leases; Contracts. The Company has no leases, contracts, or other
          -----------------                                                
agreements or commitments involving annual payments by or to the Company in
excess of $25,000 each, except as listed in Schedule 4.11. True and complete
copies of all the foregoing have been made available to the Buyer. To the best
of Seller's knowledge, the Company has complied with all the provisions of such
instruments and of all other contracts, leases, agreements and commitments to
which it is a party, and is not in default under any of them.

     4.12 Directors and Officers, Compensation, Banks. The Seller has made or
          -------------------------------------------                        
will make available to Buyer (i) the names of all the Company's directors and
officers; (ii) the names of all persons whose compensation from the Company for
the year 1996 will equal or exceed $50,000, together with a statement of the
full amount paid or payable to each such person for services rendered or to be
rendered in 1996, and the basis therefor; (iii) the name of each bank in which
the Company has an account, or safe deposit box, and the names of all persons
authorized to draw thereon, or to have access thereto; and (iv) the names of all
persons holding powers of attorney from the Company, and a summary statement of
the terms thereof.

     4.13 Litigation. Except for suits of a character incident to the normal
          ----------                                                        
conduct of the Company's business and involving not more than $5,000 in the
aggregate and except as

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<PAGE>
 
disclosed in Schedule 4.13, there is no litigation or proceeding pending (except
             ---------------                                                    
for litigation or proceedings which may have been initiated, but notice of which
has not been received by the Seller), or to the Seller's knowledge threatened,
against or relating to the Company, its properties, or business, nor do the
Sellers know or have reasonable grounds to know of any basis for any such
action, or of any governmental investigation relative to the Company, its
properties, or business.

     4.14 Leases, Contracts, and Licenses. To the best of Seller's knowledge,
          -------------------------------                                    
Seller represents and warrants that the transfer of its shares in accordance
with the terms of this agreement will not constitute a prohibited assignment or
transfer of any of its licenses, leases, or contracts, and that all of the
foregoing will remain in full force and effect without acceleration as a result
of this transaction.

     4.15 Authorization and Enforceability. This Agreement has been duly
          --------------------------------                              
executed and delivered by Seller and constitutes the legal, valid and binding
obligation of Seller, enforceable against him in accordance with its terms.

     4.16 No Violation of Laws or Agreements. To the best of Seller's knowledge,
          ----------------------------------                                    
the execution and delivery of this Agreement do not, and the consummation of the
transactions contemplated by this Agreement and the compliance with the terms,
conditions and provisions of this Agreement by Seller and Company will not (a)
contravene any provision of the certificate or articles of incorporation or
bylaws of the Company; or (b) conflict with or result in a breach of or
constitute a default (or an event which is reasonably likely to, with the
passage of time or the giving of notice, or both, constitute a default) under,
or result in or permit the modification or termination of any provision of, or
result in or permit the acceleration of the maturity or the cancellation of the
performance of any obligation under, or result in the creation or imposition of
any liens of any nature whatsoever upon the Company's assets or give to others
any interests or rights therein under, any indenture, mortgage, loan or credit
agreement, license, contract, lease or other agreement or commitment to which
the Company or Seller is a party or by which any of them or any of their assets
may be bound or affected, or any judgment or order of any court or authority,
domestic or foreign, or any applicable law, rule or regulation.

     4.17 Disclosure. No representation or warranty by the Seller in this
          ----------                                                     
Agreement or the Schedules to this Agreement, contains or will contain any
untrue statement of a material fact, or omits or will omit to state a material
fact necessary to make the statements contained herein or therein not
misleading.

     5. Representations and Warranties of Buyers. Buyer represents and warrants
        ----------------------------------------                               
to Seller as follows:

          5.1 Organization and Good Standing. Buyer is a corporation duly
              ------------------------------                             
organized, validly existing and in good standing under the laws of the State of
Delaware. The copies of

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<PAGE>
 
Buyer's certificate of incorporation and by-laws, as amended to date, which have
been delivered to Seller, are true and correct and complete and are in full
force and effect.

          5.2 Authorization and Enforceability. Buyer has full corporate power
              --------------------------------                                
and authority to make, execute, deliver and perform this Agreement. The
execution, delivery and performance of this Agreement by Buyer have been duly
authorized by all necessary corporate action on the part of Buyer. This
Agreement has been duly executed and delivered by Buyer and constitutes the
legal, valid and binding obligation of Buyer, enforceable in accordance with its
terms.

          5.3 Brokerage. Buyer has not made any agreement or taken any other
              ---------                                                     
action which might cause anyone to become entitled to a broker's fee or
commission as a result of the transactions contemplated hereunder.

     6.   Conduct of Business Pending Closing. The Seller covenants that, 
          -----------------------------------
pending the Closing:

          (a) The Company's business will be conducted only in the ordinary
course.

          (b) No change will be made in the Company's authorized or issued
corporate shares.

          (c) No dividend or other distribution or payment will be declared or
made in respect of the Company's corporate shares.

          (d) All debts will be paid as they become due.

          (e) No contract right of the Company will be waived.

          (f) No obligations except current liabilities under contracts entered
into the ordinary course of business will be incurred.

     7. Company Personnel. At the closing, the Seller shall make available to
        -----------------                                                    
the Buyer, unless otherwise requested by it, the written resignations of the
Company's directors and officers and shall take, or cause to be taken, such
action as the Buyer may request with respect to changes in directors and
officers.

     8. Conditions Precedent for Buyer. All obligations of the Buyer under this
        ------------------------------                                         
agreement are, at its option, subject to the fulfillment, prior to or at the
closing, of each of the conditions described in this Section 8.

          8.1 Representations and Warranties True at Closing. The Seller's
              ----------------------------------------------              
representations and warranties contained in this agreement shall be true at the
time of closing

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<PAGE>
 
as though such representations and warranties were made at closing and shall
continue to be true at the time payment is due under the Note.

          8.2 Performance. The Seller shall have performed and complied with all
              -----------                                                       
agreements and conditions required by this Agreement to be performed or complied
with by him prior to or at the Closing.

          8.3 Opinion of Company's Counsel. The Seller shall have delivered to
              ----------------------------                                    
the Buyer an opinion of counsel that the Company was incorporated and is in good
standing under the laws of the State of its incorporation and, that to the best
of their knowledge, there is no litigation pending against the Company which is
not listed in Schedule 4.13.

     9. Indemnification. The Seller shall indemnify and hold harmless the
        ---------------                                                  
Company and the Buyer, at all times after the date of this agreement, against
and in respect of:

          (a) Undisclosed Liabilities. All liabilities of the Company of any
              -----------------------                                       
nature, whether accrued, absolute, contingent, or otherwise, existing as of the
date of Closing excepting those listed on Schedules 4.6 and 4.11, including,
without limitation, any tax liabilities, accrued in respect of, or measured by
the Company's income for any period prior to December 31, 1996, or arising out
of transactions entered into, or any state of facts existing, prior to such
date;

          (b) Interim Liabilities. All liabilities of, or claims against, the
              -------------------                                            
Company arising out of the conduct of the Company's business between December
31, 1996 and the Closing, otherwise than in ordinary course, or arising out of
any presently existing contract or commitment listed in Schedule 4.11.

          (c) Taxes. All the Company's Taxes attributable to or apportioned to
              -----                                                           
any period on or before December 31, 1996 and Seller's Taxes (including, but not
limited to, those Taxes arising on account of the transactions contemplated in
this Agreement). For the purposes of this section, Taxes shall be deemed
attributable to or apportioned to a period on or before December 31, 1996 if (i)
such Taxes are for the taxable year or other tax reporting period that ends on
or before December 31, 1996 or (ii) such Taxes are apportionable to the pre-
Closing portion of a straddle year.

          (d) Misrepresentations. Any damage or deficiency resulting from any
              --------------------                                           
misrepresentation, breach of warranty, or nonfulfillment of any agreement on the
part of the Seller, under this agreement, or from any misrepresentation in or
omission from this Agreement or any Schedule to this Agreement; and

          (e) Incidental Expenses. All actions, suits, proceedings, demands,
              ---------------------                                         
assessments, judgments, costs, reasonable attorney's fees, and expenses incident
to any of the foregoing, to the extent that such items described in this Section
9(d) exceed in the aggregate $10,000.00.

                                       7
<PAGE>
 
     The Seller shall reimburse the Company or the Buyer, on demand, for any
payment made by the Company or the Buyer at any time after the date of this
Agreement, in respect of any liability or claim to which the foregoing indemnity
relates.

     Seller and Buyer agree that any indemnification payments made pursuant to
this Section 9 shall be treated for tax purposes as an adjustment to the
Purchase Price unless otherwise required by applicable law.

     Seller shall not be obligated to indemnify the Buyer and the Company
pursuant to this Section 9 unless the aggregate of all such indemnification
claims exceeds $50,000.00 (the "Threshold"), in which event the Seller shall be
liable for all amounts in excess of the Threshold.

     10. Brokerage. The Seller represents and warrants that all negotiations
         ---------                                                          
relative to this agreement have been carried on by him directly with the Buyer,
without the intervention of any person, and the Seller shall indemnify the Buyer
and hold it harmless against and in respect of any claim for brokerage or other
commissions relative to this agreement, or to the transactions contemplated
hereby, and also in respect of all expenses of any character incurred by the
Seller in connection with this agreement or such transactions.

     11. Purchase for Investment. The Buyer represents that its purchase
         -----------------------                                        
hereunder is being made for its own account for investment, and with no present
intention of resale. All stock certificates presenting the shares purchased
under this agreement shall be endorsed with the following restrictive legend:

     The Shares represented by this certificate have not been registered under
the Securities Act of 1933, and said Shares may not be offered or sold and no
transfer will then be made by the Company or its transferee except in compliance
with the Securities Act of 1933 and the rules and regulations promulgated
thereunder.

     12. Nature and Survival of Representations. All statements contained in any
         --------------------------------------                                 
schedule, certificate or other instrument delivered by or on behalf of the
Seller pursuant hereto, or in connection with the transactions contemplated
hereby, shall be deemed representations and warranties by the Seller hereunder.
All representations, warranties, and agreements made by the Seller in this
agreement, or pursuant hereto, shall survive the closing and any investigation
at any time made by or on behalf of the Buyer.

     13. Benefit. This agreement shall be binding upon, and inure to the benefit
         -------                                                                
of, the legal representatives of the Seller, and the successors and assigns of
the Buyer. Without limiting the foregoing, the Company's rights hereunder may be
enforced by it in its own name. In the event that the Buyer causes the assets
and business of the Company to be transferred to some other corporation, the
rights of the Buyer and of the Company hereunder may be enforced by such other
corporation in its own name.

                                       8
<PAGE>
 
     14. Construction. This agreement is being delivered and is intended to be
         ------------                                                         
performed in the Commonwealth of Pennsylvania, and shall be construed and
enforced in accordance with the laws of that state.

     15. Notices. All notices, requests, demands, and other communications
         -------                                                          
hereunder shall be in writing, and shall be deemed to have been duly given if
delivered or mailed, first class postage prepaid, if to Seller, at 2171 Sandy
Drive, State College PA 16803, Attn: Douglas R. Colkitt, or at such other
address as he may have furnished to the Buyer in writing, or, if to the Buyer at
3754 LaVista Road, Tucker, GA 30084-5637.

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

     17. Approval of Buyer's Board of Directors. This Stock Purchase Agreement
         --------------------------------------                               
is contingent upon the approval of the disinterested members of the Board of
Directors of the Buyer, which approval shall be obtained prior to Closing.

                                       9
<PAGE>
 
In witness whereof the parties have duly executed this agreement.



                                         /s/ Douglas R. Colkitt
-------------------------------          ---------------------------------------
WITNESS                                  DOUGLAS R. COLKITT, M.D., SELLER



ATTEST:                                  EQUIMED, INC.



                                         BY:  /s/ Larry Pearson
-------------------------------             ------------------------------------
(Assitant) Secretary                        LARRY PEARSON, PRESIDENT and
                                            CHIEF EXECUTIVE OFFICER


     The Company hereby consents to the transactions described in this Stock
Purchase Agreement to the extent such consent is necessary and joins in those
representations and warranties in Article 4 related to the Company.

ATTEST:                                  NIXON EQUIPMENT CORPORATION
 


                                         BY:  /s/ Douglas R. Colkitt
-------------------------------             ------------------------------------

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