So heres my situation.I don't have a mortgage or rent to pay and I have found a property thats in a good area and is a pretty good house. 3 bedroom really 4 but wold make the 4th a bathroom I think its down stairs and theres no bathroom down stairs.

It is a 2nd story apartment and they assured me he can do stairs so I don't believe there will be many apparatuses necessary. 

It is unlikely that your down stairs tenant will file a complaint against you although they will likely ask to be released from their lease.

I keep a quart handy for those wonderful occasions (easier to handle and lug up stairs).I would get to know the folks at your specialty paint store by name as they can be a good allies, too.Best of luck!

Put in a non load bearing wall with a locking door at the base of the stairs to separate the two spaces.

and the stairs go right up the middle of the house, with a door closing them off...  

They are big on things like old wooden stairs without railings, chimneys in need of repointing, lack of heat or electricity, commercial units in residential buildings (even though it is allowed it is really hard to convince them of that).  

I also don't think assisted living is a good option since accessibility with all the stairs is an issue.

Consider using green board (water resistant) for bathroom walls and cement board (water proof) for walls around the tub & shower.

After using a 10-pound sledge hammer for way too long trying to remove the cinderblocks, re-bar and anchor cement, I thought "There must be a better option!"