If you want to be ADA compliant, I'm sure there's a lot of info out on the InterWebs about that, but I don't recall the topic coming up here before. A good point has been made about the lift; no point in being disability-friendly if your friends can't get on the bus with you. Plenty of them out there, but it will be something you can't move on your floor plan and will have to design around. Also any wheelchair bus will have a lot of tiedown rails available, and will need to keep that area clear so that they may be used. Lots of square feet used up there.
I can't see "ADA compliant" and "24" walkway" in the same thought, myself. You'd have to carry one of those skinny wheelchairs the airlines use to get folk on/off the plane.
Of course, if all you need is Braille signage, you're home free!