I think a cummins repower is in order. The 6BT should bolt right in. A 4BT would do even better mpg wise, but, it might be a tougher fit. I think the 727 would be up to either one, just make sure you have a huge tranny cooler.
Personally, I'd just find something like this....
2003 F.o.r.d E.3.5.0 passenger bus diesel 9 passenger
The nose is already pretty sleek and the wheelchair lift would be handy for loading your 'bots. BTW, could we have a pic of your rockstars? just tack a nice boat tail on. I would think that 20 mpg would be doable.