Most Type 'A' buses came on the Ford E-350/450 or the Chevy/GMC G-3500/4500 chassis.
Five different diesel engines were available in those chassis.
- 6.9L/7.3L/Powerstroke--great engine, reliable, long lived. Available in Fords with the E4OD/4R100 transmission which were not the best.
- 6.0L/6.4L/6.7L/Powerstroke--so, so engine with reliability problems due in large part to the emissions systems. Available in Ford products and still mated to the 4R100 transmission.
- 6.2L--an okay engine low on power but ultra reliable. Available in small GM and Workhorse chassis. Most common transmission was the THM-400/700R4 which was also an ultra reliable and long lived transmission.
- 6.5L--turbo version of the 6.2L but different enough that parts rarely interchange with the 6.2L. Some of the parts chassis application specific and are dealer only. Which means those parts are no longer available. Aftermarket support for the engine is great for the light C/K truck line but zero for the G-chassis line. Early versions were paired with the THM-400/700R4 with later versions getting the 4L60, 4L80, or 4L85.
- 6.6L Duramax and Allison transmission--great engine and transmission but virtually non-existent. It was a $14K option that few operators chose.
Gas engines have run the gamut from Ford small blocks like the 305/5L and 351/5.8L and the big block 460 to the new technology V-8's and V-10. The 460 was a gas hog and was the impetus for so many choosing the diesel. Going 4-6 MPG to 10-14 MPG paid for the difference fairly soon.
Gas engines from GM have also run the gamut from the 4.3L V-6 to the Vortec 7400 or 8.1L.
Probably the best running engine of ALL of the engines available in van cut-away chassis has to be the 350/5.7L/Vortec 5700 engine. They routinely and regularly see 300,000+ miles with nothing more than oil and spark plug changes. The follow on 6.0L is even better.
Collins/Mid Bus have been the small bus builder for more than a generation. They have pioneered most of the development in that part of the industry. Thomas Minatour, Blue Bird mini-bird/micro-bird/Girardian, Carpenter Cadet, Wayne Busette, and Superior were all okay buses but usually used scaled down versions of full size buses. This wasn't bad but sometimes the transition to a smaller scale didn't work out quite as well as the makers thought it would.
Regardless of body OEM all Type 'A' and Type 'B' buses suffer the same problem in that all of the, regardless of brand, have a lot more bus on top than truck underneath. It is very easy to overload the small buses to the point where it could be dangerous. Most empty are at 80%-90% of their total carrying capacity. For most the limit is how much tire is under the bus. 16" LT rated tires can only support so much weight.
Good luck on your decision making process. I hope we might be able to help you on your quest.