Oh, we are talking about a van chassis? Ok. Totally different situation but not much better as far as diesel engines. GM was partnered with Detriot Diesel at the time assuming we are talking about 80's or 90's.
They produced the 6.2liter and 6.5liter diesel, which was basically just a diesel version of the standard 350 gas engine. The 6.2 was the earlier one and was very underpowered. They both are underpowered but the 6.2 was worse.
If I was going your particular route looking for a half bus I would not even bother with those early GM diesels unless it has the Duramax 6600 diesel after 2000. \
I know it would not be friendly to your wallet but I would look for a gas engine in a GMC or Chevy half bus. Preferably the 454 (7.4liter or just the standard 350(5.7liter). Both are great engines. The 350 is tried and true and just about bulletproof.
And if they are the Vortec version. Even better.
Alot of those half buses came in Ford Chassis's as well. Again depending on the year, you should look for the fuel injected 351 5.8Liter, 5.4liter or the 6.8liter V10.
If you find a Ford bus with a Diesel between 1995 and 2002 for a good price GRAB IT! It will have the much desired 7.3 liter Powerstroke which was a great engine in that application. Its too bad they scrapped it in 2003 and replaced it with the piece o' #$% 6.0 diesel.
If you have $4,000 you can have this bus.
. Yeah, those Diesels are loud, smoke and underpowered. You will be able to see a label in the engine compartment that will show you what engine. It will say either 6.2, 6.5, 5.7 etc... 
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