I was thinking the same as cjchasman, I dont know if you own your house, or rent, but you could rent your half of the house, keep your mortgage on that place and pay it off as a landlord. Move up to the property, but like everyone said get things in writing. Maybe a clause that gives you 3-6 months to find another place to live if something happens to the old man or the situation.
I dont know if your a praying man, but it always helps me to make big decisions. If in doubt and you have reservations that you cant shake you may want to wait. Just a thought, good luck either way; and if it works out to be true, it sounds like a great deal.
"If Darwin saw the same sunrise and sunset, that I see from a duck marsh, or my tree stand; then he would know there is a God, and that He created all of it." -Ford-