RE: Looking for a good running/rebuilt Cummins NHC 250 Diese
Ditto above.
I bought a 'in the can' motor from Vermont Commercial Salvage, for my m51, I paid more for the motor than I paid for the whole truck.
A 290 cummins is a common engine at truck boneyards, sometimes they can be had complete for as little as $500.
they have the oil cooled pistons, and can be turned up for more HP, even if you decide to strip off the turbo and intercooler to make it look like a 250.