Hi Just a word from Europe where the Sprinter and future Fiat things might come from:
The emissions regulations are now more strict over the pond and therefore no particular engine mods should be undertaken. For an example the Ford E series diesels are not accepted anymore even as special import as they do not manage to comply with the latest diesel emission norms.
That being said most euro vans are somewhat underpowered apart from a few. The V6 sprinter is not bad and some VW vans are quite powerful but remember the high mileage you guys put on your vehicles due to greater distances and comfort of a low revving engine are second to none.
I drive quite a lot for European standards and I couldn't imagine a whizzing little TD engine with 2t (or more) to drag around... let alone go off road...
On the Sprinter front you should be ok for parts as the Merc version is basically one of the most popular van around over here... and seems gaining terrain fast in the US...
But as others have wisely said it is all very subjective...
Good luck with the choosing and designing