I would say a seasoned bus photographer doesn't really have the
need to get a Digital SLR camera. It's way over the mark of taking
good bus photographs.
Though I own a Canon EOS350D, I hardly use it as I don't really
find the need to do so. I would always be using Canon Powershot
A510. Though it's a consumer camera, it produces pictures that
satisfy the OK level.
IMO, if one wants to take bus pictures with a better camera,
perhaps prosumer cameras like Canon S5IS is good enough.
For starters wise... Canon Powershot A500 series can be a model
that is worth to commence with.