WDW has "featured" vendors who are allowed to utilize Bell Services for pick-up/return; other vendors are allowed to rent to guests at resorts and will be met in-person. There are lists of both featured and non-featured vendors in post #2 of the
disABILITIES FAQs thread pinned near the top of this forum, along with some feedback from other DISers.
The WDW buses can accommodate at least 2
ECVs or wheelchairs, some can take 3. Depending on the bus route, there may already be 1 or more ECVs/WCs on the bus when it pulls up to your stop.
Enjoy your vacation!