If I were in your shoes I would give the Isuzu NRR a hard look. They are the same physical size as the common NPR trucks but with a GVW around 19.5k lbs, versus the NPR's GVW of around 10k. There is also a NQR that is somewhere in the middle. If you look long enough you can find a NRR with the 6cylinder diesel. The 4 cylinder has a reputation for being rather underpowered for even the NPR.
The problem with a f650 size platform is that it will be exceedingly wide for what you want to do. The NRR will really be a little too wide, but much closer than a medium duty truck.
