[kaisen]

It's not so much about the engine, but the Mercedes Sprinter chassis. If he really wants a 'B' (stock body) and not a 'C' chassis-cab with RV body, then the Sprinter is the way to go in terms of configurations. Taller ceilings, wider/longer bodies, dual rear wheels, etc are all available in complete-body stock form.

That said, if I were giving advice, I wouldn't necessarily buy a Sprinter. Horrible body integrity and rust issues. Not necessarily a better chassis. Not really a better engine (although all you German diesel fans will argue). Not a better transmission. Certainly not better hubs/bearings/brakes/etc. And you get to pay MUCH more for it, and have a harder time finding parts that you also get to pay more for.

But if it is Class "B", the Sprinter diesel widebody dually is the way to go.

Otherwise get a Ford or GM, pref. Class "C" or hybrid "B" (narrow-body but built from chassis-cab). The GM Duramax/Allison powertrain is superior to the little Mercedes.