You can see one example of an agreement to sale draft here. Typically sale agreements are tailor made for specific to the property. Ask a good lawyer let him draft for you. Tell him all your concerns and fears and conditions. Tell how you will like to make the payment. Basically the schedule of the payment. Most importantly what if the transaction fails, that is also crucial.He should be able to capture all these in legal language.
A good and fair sale agreement between buyer & seller builds enormous trust between the parties. This will keep you tension free and you can complete the ownership transfer in a smooth manner.
Sammy