I have looked at the 2-1/2 & 5 ton trucks off and on for a while. Even bid on some, but without success. I've again started looking and have noticed more 900 series trucks now. I like the 923's. I there anything in particular I should look for and how reliable are they? I may wind up trying to pick one up.
I don't see in your question where you asked about license and registration. However since it was mentioned I'd suggest taking a look at your states registration and licensing requirements.
You didn't mention where you were looking, GL, truck dealer, or private party.
Your best bet would be a dealer or private party. At least then you have some sort of history, and in most cases can drive, inspect and stew over a particular truck. Not all of that is possible with GL.
As far as inspection I would download the manual either from the resources here or liberated manuals, and print the inspection checklists. Use that in your inspection. Look for things rusted, loose, bent, broken and missing items. Note those items on the checklist.
After inspection which should include fluid levels, and belt tension inspection. Ask to operate the truck or at a minimum start, and run until it reaches operating temperature.
Listen during the start up, and run. Things to pay attention to are hissing air and abnormal engine noise. The NHC-250 has a familiar engine rythem, there are inspection videos produced by the army on the M939 trucks. A set of three in fact, and can be found on you tube. These are very helpful to the new operator and will at the very least let you hear the engine run how it's supposed to sound.
If you can operate the vehicle, ensure the tire pressure is correct on all positions. Listen and feel how the truck operates. Loud pops, growns and squeals aren't normal and should be noted and investigated.
Things that would be a concern to me are worn tires, cracked belts, lots of rust and missing parts.
Aside from requiring very big tools they are pretty easy to work on, for the most part.