Kami,
I would suggest Magnolia Terrace or Oak Bend, both in the Magnolia Bend section. In the Alligator Bayou area, building 14 would be my top pick. All three of these are close to the main building, eating areas, and themed pool. Magnolia Terrace is also close to a quiet pool and a bus stop.
All that said, I very strongly suggest that your mother rent a wheelchair. I'm not sure if you've ever been to Disney before, but there is a lot of walking involved. A whole lot. This seems to be coming up a lot lately, but it bears repeating - my family often walks 10 or more miles per day on a Disney vacation. From what you've described I can't imagine your mother would be able to do that. I'm afraid that without the use of a wheelchair she'll be spending the bulk of her time resting on benches or in her hotel room, neither of which is very fun.
I know that renting through Disney is expensive, but we've rented push chairs and ECVs both through
www.walkermobility.com and have been very pleased. Their rates are much lower, and they also deliver and pick up right at the resorts, so you don't have to fool with transporting the chair. It's very easy, and worth every penny of the rental rate when you consider the alternative.
My mom had foot surgery just before her first trip to Disney and had to spend our entire trip in a wheelchair. While I know it wasn't her ideal, it made it possible for her to still go and have a great time with us. If she hadn't rented the chair, she would have been miserable. It's just a lot of walking, no way around it. i don't mean to sound discouraging, but I want you and your mom to know up front what to expect so you can make an informed decision.
Best of luck!