Purchase Agreements

When signing up a property that I will present for a short sale, are there any particular clauses that I should include in the Purchase Agreement? My exit strategy would be to wholesale to another buyer using a double closing. Also, when signing up the deal is it better to use a short 3-page purchase contract or a more lengthy (10-15 pg) one? Where can I find a purchase agreement that I can also edit to insert these clauses. Thanks.