If you decide to do this speak with their loan officer to make sure the agreement you work up will fly when it comes time for them to pay you in full. I have never seen one of these work out, never done a take out loan for one and I have closed thousands of deals. See the link below for more info.
NMLS # 6395
Financing Kentucky One Home at a Time
You and your tenant might also find my article, entitled Understanding the Lease Purchase, at this link http://db.tt/RFJuKqn helpful.
There are many books and local resources that can help. To answer your direct questions; 1) your benefit; typically the buyer/tenant is responsible for most repairs and expenses, you probably will get a higher than market selling price, you should get a nonrefundable option fee, you probably will get a good income stream with the interest charge. 2) i would have a local lawyer draft up the paperwork, while being fair but leaning towards your benefit. 3) i don't believe it will hurt your credit "per se" but your name is still on the mortgage thus limiting your investment oppts.
call me and we can talk more, Tim Dolan 603-320-6495