Something else to consider, the GM full-size trucks/Suburbans have much more aftermarket support than the vans. Also, if you wanna work on it yourself, the vans are very tight under the hood & dash.
I've looked into just getting a set of headers for the asthmatic 350 in my '98 GMC 3500 dually utility van, but no one makes them, and going custom probably wouldn't be worth the hassle.
The MY2003+ vans come with an LS-based motor, or you could try to find a 2007+ 3500 with the rare Duramax (de-rated to ~250HP & ~460 lb-ft).
I don't know how much weight you need to pull, but the Mercedes/Dodge/Freightliner Sprinter diesel vans are nice, they have 78 inch stand-up height in back, rated to tow 5000-7500 lbs, and get good fuel economy. I would've made one into a mobile shop, but my GMC came with a shop in back already installed:
