What is the best agreement to execute to purchase a property in the future that we will be making all payments on until then?
You and she can enter into an agreement whereby if you make the payments you describe for the specified period of time, she will then sell you the property at the end of that period for an agreed-upon price: either a price you agree to now, or the then-fair market value (e.g to be determined by appraisal or other mutually agreed upon method) less the value of the payments you make in the interim (i.e. you will be credited those payments). As long as it is in writing, such an agreement is enforceable.