Keep an eye on craigslist, and set up an email alert for the keyword "School Bus".
For the most part busses on craigslist are overpriced, but occasionally a sweet deal will come up, and if you have an email alert set up you will be the first to know.
I am sure you have these sites already marked but just in case:
Public Surplus: Government Surplus Auctions
I got my buss off of:
and it was a good experience using them.
From my understanding its probably best to avoid CAT if you want an easy engine to work on. I think a Cummins engine would be a good bet. I am not experienced with diesel mechanics, and have been finding it very accessible, and a good engine to learn with.