You're probably stuck with a Real Estate Attorney:
There aren't any FORMS because all the terms and details would be different:
You would be at the mercy of the Seller; he would dictate the terms; high selling price, high deposit/down payment, low monthly payment applied to option, draconian terms if the option wasn't exercised on time, you responsible for maintenance and repairs, etc.
This is not a good thing for the Buyer!
And the kicker is that it wouldn't help your Credit Score, because as a private individual, he wouldn't report to one of the Credit Reporters.
It's the ultimate Caviat Emptor!