serious ? re 1st deal

I have my 1st deal on the table…probably…
I found a house that was FSBO and it didnt sale, now they want to rent it…I approached them about lease/purchase to me subject to their mortgage. of course, I want to lease/option it out to someone else.

My ? is how do I structure the deal w/ the present owner where they don’t have any liability to me re: equitable interest and 2nd’ly how do I structure an agreement w/ a TB using an option, where I don’t have any liability to them re equitable interest? I’m guessing there are specific contracts that will address these issues, but where do I find them and or set this deal up? Most of the books I’ve read don’t really address the issue of equitable interest.