If you are staying at one of the WDW resorts, the bus is the best way to get to MK because it drops you off right at the turnstiles to get into the park.
As pjpoohbear mentioned, the handicapped parking at MK is a long walk to the TTC where you need to catch either the boat (a longer walk) or the monorail (a walk up a steep ramp to the monorail station at 2nd floor level). There are no trams to get from the handicapped parking area to the park entrance. There are some "courtesy" wheelchairs at the handicapped parking areas that you can use to get to the area where you can rent a wheelchair. There is no guarantee that any courtesy wheelchairs will be available in the area when you ge there, though.
If you can walk short distances and step up to get into the tram, you will be better off (less walking) to park in the regular parking lots and use the trams. The parking lot trams drop off and pick up close to the TTC at MK.
You might also want to look into renting a wheelchair or
ecv. There is information in the disABILITIES FAQs threadd near the top of this board.