Having been in the 'band business' in years gone by, I think my personal suggestion would be to find a good road touring bus like an MCI, Eagle, or Prevost.... These things are bigger, have a smoother ride, nicer seating, and have those huge pass-thru cargo bays underneath for the equipment and stuff.
Be that as it may, if Skoolie is what you are interested in, it certainly would be better than a van. The bus should be able to handle all the equipment, you may have to remove a few of the rear seats for storage space. The church bus sounds good, but there may be better deals out there. Like those 26 Thomas transit style school buses that http://www.sonnymerryman.com/
is selling for $2000 each. You might even get lucky and find one with a storage compartment below.
My bus came from the same area as those that SonnyMerriman is selling, and mine has little to no rust on it...mine came from City of Harrisonburg Va....