Under the ADA law if the hotel provides transportation for able bodied people they must also provide it for those in wheelchairs. I would call the hotel to notify them that you will be needing accessible transportation so they can have it all set up when you get there.
They are obligated to provide accessible transportation that is comparable to what they provide to other guests.
Depending on the hotel, what they provide to other guests may not be that good to begin with.
- Some have 'free' bus service, but have extra charges, like a 'facility fee' (or charge for parking) that is not listed in the rate per night for the room.
-As was already posted, some share buses with other resorts, so you may be spending a lot of time traveling. Even if every bus has an accessible spot or more, it could be occupied when the bus arrives. This can happen at a WDW resort, but with buses coming every 20 minutes or so, your wait for the next bus won't be long and the driver can call to get a bus routed to pick you up.
- Some provide only limited pickup times, so you have to see the park on their schedule, not yours. If you miss the last bus, getting back to your resort is your problem and your expense. Accessible taxis are available, but you may have to wait longer.
Considerations about renting a car:
- check around for discounts. You may be able to get a AAA discount, a discount by renting thru your credit card company, the Costco or Sam's Club if you are a member.
- check into different pick up places. Some have pick up and return at one of the a Downtown Disney area resorts that may be less expensive than doing at the airport.
- if you are not staying at a WDW resort, you will be paying a parking fee at the parks each day. Annual Passholders get free parking, but depending on how long your stay is, getting an annual pass to take advantage of that may not be cost effective.
Check into other close resorts that have better service.
- Swan and Dolphin have quite a few discounts ( I've seen people posting about discounts for teachers and nurses). They are walking or boat distance from 2 parks and use WDW transportation for the others.
- also check into Downtown Disney area resorts.
I don't know what on site resort you were looking at, but do look at options like the a Family Suites at the Value Resorts and the cabins at Fort Wilderness.