whats the objective and what are the requirements? theoretically you can covnert anything into anything... when you say transit bus are you wanting a front and mid / rear side door? kneeling front? stick to auto? Air-conditioning? all things that to me are pretty straight forward.
It is to take an old school bus and turn it into a public service transit bus. I know that some things would be calling for adding a side exit door, installing an A/C, and the changing of the seats. St. Louis Metro's Call-a-Ride vehicles are mostly junk. And an old Carpenter from the 1960s would be a much better bus than a van.
what are you going to use it for? actual public transit by a transit agency? private tours? reason i ask is will it have to be completely ADA compliant with all of the modern safety features required by a true Public transit company? or if it is simply a private tour coach. (ie hop on hop off scenics).. there are different requirements for both when it relates to needing to handle wheelchairs and such.. private trolleys in many areas dont need to be fully ADA..