in general terms, the inline 6 diesel engines (DT-466/CAT-3126/Cummins 5.9 / Cummins 8.3). will have more torque than the V-8 Diesels (T-444E(some call it a DT444), VT-365, CAT320
ECCB has it right.. the general consensus is that reliability seems to go downhill on many of these enfgines in the 2004-2005 model year range and up.. and inthe 07/08. and newer range full-on emissions were required which further increased complexity and seemed to reduce reliabilty..
short busses - there are different types of shorties.. alot of drivetrain options depend on which type you are looking at..
there are the Cust-away shorties.. which the front part looks like a standard chevy or ford Van.. and the back looks like a school bus.. there are various drivetrains all of which you can generally buy parts at your local auto store for.. in the chevies the gasoline 6.0 litre is a favorite as its reliable and parts are everywhere.. you'll likely have a GMC 4L60E or 4L80E overdrive transmission.. Duramaxx and 6.5 litre were diesel options for these in the 00+ years.. if you get the duramaxx you'll have an allison 1000 transmission..
in the ford cutaways you'll most often find a Powerstroke.. stick to the pre 04 years and you'll get the venerable 7.3 which is generally a reliable diesel in these vans and is pre emissions.. the 6.0 diesel can be a reliable engine, however its not for thew faint of heart.. as in its early years for sure, it had serious issues well documented on the internet.. and requiring (Bullet-proofing) to make it good.. if you have no mechanical knowkedge tend to stay away from the ford 6.0 diesel..
now the other type of shorty bus is a CE or conventional.. which looks just like a big bus but is shorter.. International and freightliner are the 2 main chassis you'll see.. and the bodies were made by a myriad of manufacturers over the years.. these tyype busses will have truck drivelines in them. and are where you'll find your truck engines like the DT466 or CAT3126. and others.. thje allison transmision options were many depending on the type and year of the bus.. the allison 545 was the lowest cost option. and was a fantastic transmission for in-town bus route use, however on the highway esp in the mountains it can be lacking. due to it not having the locking torque converter... in a shorty bus they can be fun, esp if you keep the fluid fresh and install a high performance transmission cooler....
the T-444E is a very similar engine to the ford powerstroke 7.3. international built them for ford.. it works well in a shorty bus. but lacks in power if you buy it in a full length bus.. 2004 was its last year in conventional school busses..
be patient, read threads about real world results of various busses and thse on here.. people taking trips often post MPG results, accounts of how their bus does, etc.. those are the real world experiences you want to learn from before you go out and buy a bus..