All the advice given is excellent and should be taken. Here is another reason to look carefully at these type of deals especially when a mortgage is in place. If the lender does not allow for it's loan to be assumed or carried-back and becomes aware of the new situation, (which is usually mortgage fraud), then it is likely that in the small print on the original loan the lender can call the note. If neither parties, you living in the property making the payment or the original owner/note holder cannot pay the full amount remaining on the mortgage at the time the note is called then guess what happens--the bank collects the collateral=the home, and you get left out in the cold.
Buyer beware is very applicable here. If you really feel this is the right option for you then look at properties where the owner has title and no mortgage is involved. If there is a mortgage than look at assuming it for better protection.
Realtor, GRI, CSSN
John Hall & Associates