I would say try Old Key West in a Grand Villa. They sleep up to 12 and is one of the biggest (sq footage wise) Grand villas on property. They are also on one floor where others, like Saratoga are built as a two story unit.
I would think that OKW would also be the cheaper of the villas. I know that they are points wise for DVC members.
There are only 2 OKW Grand Villas that are accessible. Both are on 2 floors. We stayed in the OKW GV in building 48. You enter on ground level. The kitchen, laundry room, living room and large dining room are huge and fully wheelchair accessible with a roll under sink in the kitchen.
There is a queen size sleeper sofa in the living room. They may have also added the chair that folds out to a twin size sleeper bed. There is a hall bathroom with a shower, which is not accessible (very small and not able to get a wheelchair in to use it).
The master bedroom has a King size bed and the bathroom there is accessible with a roll in shower and grab bars by the toilet.
There are 2 other bedrooms, with 2 beds in each and 2 bathrooms upstairs. Those are not accessible and the only way to get up there is by the stairway.
The other accessible GV at OKW is in one of the buildings numbered in the 60s. Since that building has elevators, it is possible to enter the top floor and the bottom floor from outside and roll in. But, you cant get from one level to the other inside the villa, as far as I know. I have not been in that GV, so cant tell you more about it.
There would be LOTS of room on the ground floor though for wheelchairs.
All the 2 bedroom villas at OKW were recently renovated and they added a lounge chair that makes into a twin sleeper. Previously, the 2 bedroom units were listed as sleeping 8 - I dont know if they now allow 9 because of the twin sleeper. If so, that might be an option for you since there is lots of space and it may be possible to have Housekeeping remove some of the furniture if you dont need it for seating (like a love seat in the living room and the dining room chairs).
I have heard that Boardwalk has some GVs that are on one floor, so possibly someone knows something about that.
The other consideration is buses. I dont know how you planned to get to the parks, but that would be a consideration since each bus carries only 2-3 wheelchairs (with the majority only able to carry 2).