Keep in mind that there are many buses on the used market that have lift gates installed, should that be a concern for your client/friend.
One of the first things I did was to set up accounts on GovDeals.com and PublicSurplus.com and set up searches that emailed me whenever any new listing contained the magic words "school bus". Easy to do and it was where I got my first training in bus manufacturers' names, engine and transmission manufacturers and model names, what the various buses look like, etc. Depending on how new you are to this world, you might be a little overwhelmed by the vocubalary for a while.
Also, read a hundred or so bus conversion stories ("threads") on this site. The more total postings, the better the thread IMHO. You will end up knowing how and why you should remove all the old insulation, how to remove and replace windows, where to pound on your brakes with a hammer
, and a myriad of other necessary life skills.
Good luck and welcome!