First, all Disney Transport buses are low-floor, with no steps to the front half of the bus. All can "kneel" on request to shorten the single step in, although occasionally the kneel mechanism might be out of order. The ramp at the rear door can also be deployed for riders who have trouble stepping up, not just for wheelchairs. Sometimes high-floor charter buses are brought in to help out at busy times between the parks & resorts, bur never on the Fort internal routes.
The Minnie Van option shows up automatically when you're on WDW property. To see what the fare would be while you're off property, you'd need to enter both a pick up location and a destination in the Lyft app. The destination would be Hoop De Doo Revue. The Minnie Van fare is under "Other". I just ran it and the fare came up at $20.