Caribbean Beach, Alligator Bayou at Port Orleans Riverside and Old Key West all have buildings with 2 floors which do not have elevators. As Michigan posted, they are not required for 2 floors or less.
If you want to stay at one of those resorts, just request first floor for medical reasons. Many people actually request 2nd floor because they like the view or don't want anyone above them. So, even without a request, chances are 50:50 that your assignment will be first floor anyway.
POR has evevators in the Mansion buildings and OKW does have 3 newer, taller buildings with elevators.
All other resorts do have buildings with 3 or more floors and do have elevators.