The Polynesian would be a good choice--none of the rooms are too far from the GCH.
In all honesty I don't always think the monorail is such a great thing. Remember, to go to Epcot you still need to transfer, and those are long, steep ramps to climb at the TTC.
You might also want to consider renting at the BCV, they are fairly close to Epcot, and if you cut through rahter than walking all the way around inside it's pretty close to the bus stop as well. The villas are large and comfortable, and offer much more room than a regular room, and all the amenities of home. If I were taking my mother this is where I would stay, she could be comfortable relaxing in the villa if she wsa too tired or sore to manage the parks anymore.
Anne