The Parterre Place building is the closest to the front lobby and the main bus stop.
Oak Manor is also close. It is no longer a smoking bldg. We stayed there in December.
What counts is the distance to the bridges over the river since you cannot walk like the crow flies.
To some degree, it also depends on which side of the building you are on.
In your reservation request, say something like 'Need room close to lobby because spouse cannot walk far.'
If they know the reason then it will help them assign you a close room, regardless of the bldg.
We have stayed in all four buildings.
I think that you will be OK in any of the buildings except Arcadia House, which is the fartherest.
All four Mansions area buildings have elevators.
Good luck.