When it comes to subto, make sure you have yourself covered when it comes to the due on sale clause. When the deed is transferred to you in exchange for paying their mortgage, the bank will most likely come calling for the full amount of the mortgage since you aren't the one who qualified for the loan. You can protect yourself with due on sale insurance which is usually about 1 % of the purchase price and will pay the bank for the amount called and provide the same exact terms to you as the second mortgage. You could also do a lease option for the mortgage amount with the choice of purchase being $0 at the end of term, keeping the deed in the owners name and avoiding the due on sale clause. However, this option prevents you from writing off depreciation. The third option, which I think is best, is when you use a land contract or agreement of sale, which transfers ownership rights to you. I would also make sure the seller signs an affidavid that they acknowledge the mortgage will stay in their name. It also is worth it to pay for a title search to ensure no other leins besides the mortgage exist on the property so the payoff is equal to the amount the seller is saying.
