These agents are giving you good warnings about this type of situation. There is great risk, and that is why there are no TREC promulgated forms to fit it.
From the lender's standpoint, I need to add that any "rents" that the seller would give you back as credits towards your down payment are only allowed to the extent that they exceed the fair market rent, as determined by the appraiser. You would need to provide cancelled checks to prove your payments. Of course, he can pay closing costs and prepaids, again following the guidelines of your mortgage type.