I have seen a lot of Cummins 5.9/ISB and 8.3/8.3L in buses and would not hesitate in getting a bus with one of those.
There are quite a few Cat 3116/3126's out there. They are good engines but tend to cost more to maintain than the others.
For most practical purposes you want to stay away from Detroit 2-cycle engines unless you or someone you know really "knows" a DD 2-cycle. They are good engines but they have quirks that will destroy them if you don't know how to care and repair them properly.
I have seen a few buses that have had DD Series 50/60 engines in them. Those are some of the best engines ever built but I have never seen one of them in a school bus.
As for the Cat, Detroit, Cummins, and IHC V-8's, I would pass on any of them. For the most part they are lower in performance in HP, torque, fuel mileage, and longevity than the I-6's from the same OEM.
You will be hard pressed to find a mechanical DT466 in any RE that came from the factory that way. IHC didn't have them available in OEM supplied chassis and didn't start to offer them in the Ward/Gensis/IC RE until the mid-90's which by that time had started to be electronically controlled.
As you go shopping be aware that all of the manufacturers prior to about 2003 had 9" windows and 72"-75" headroom as standard. 12" windows and high headroom options were available but were not very common. Those 3" come in really handy if you are not height challenged.
Good luck and happy trails to you!