If they're all Sprinter based models you're looking at I don't think much will differ for
gas mileage unless one is significantly lighter than the others. Light, in a Sprinter,
could mean handling issues particularly in cross-wind situations. The Sprinters (I think) are also
naturally a bit taller than those built on Ford/Chrysler/GM cargo vans. The non-Sprinter vans may
feel a bit more stable with their width and shorter height. Makes them less of a sail on the road.
Having said that, the Sprinters haven't had their roofs cut off and replaced with a fibreglass lid.
Might be an advantage structurally, or over time, for leaks, and cracks? I don't know.
New or used? I think class B's generally hold value better than some other types of vehicles,
so you might find a used bargain somewhere like RV Trader. Buy one a few years old, after the
big initial depreciation hit has already happened. We did.
I have no idesa what a mobile office might entail, so you'll have to look at the internal layouts
to decide what might work best, for what you see as essential for a rolling cubicle.
If all things are pretty much equal, I usually let price decide.
Hope this helps.