In your case, mileage will be irrelevant. The engine is going to rack up lots of hours. The driveline, not so many. If you can find a truck with a newer rebuilt transmission and rears they'll outlast the rest of the truck.
I'll vote for the expedite rig as well. You can find straight trucks with a small reasonable sleeper in both the medium and semi sizes. I'd figure out your wheelbase and how old you're willing to have it and then keep an open mind about engine size and brand and look for an expedite rig that fits your needs. Pull the box oof and fab your body. I think a medium duty would do it (not an isb, but at least in isc, cat C9, not c7, etc. 8 liters plus). But having a bigger engine but running at low capacity doesn't seem to hurt fuel mileage much to be honest. We have an ISC and we flog it and it gets the same mileage as the ISX trucks we rent.
If you're going new just tell your specs to a dealer and they'll get you what you need. Also, the dealer nearest your yard that is decent is often the best choice, due to turnaround time on parts, all other things equal.