I hope you find something that works out really nicely for you.
I'm not sure how many 4x4 E350 diesel 15 passenger vans exist, much less how likely you are to find one. The 4x4 was, of course, an aftermarket conversion.
If it were me, I might also consider the current Transit vans—which don't have quite the same tow capability, but should be adequate for the trailers you're looking at with a bit of care—or the Nissan NV vans. Both have much nicer seats in the back than the usual Ford E-series ones. If you're thinking of spending a good amount of time actually on the road, that can make a big difference.
Just my unsolicited opinion...maybe not even worth what you paid for it...;)
