Welcome, you sound somewhat serious.
Check threads here for any CA title issues specific to 5 ton MV's. An SF-97 is not the same as a title, you want to make sure you can get a road legal title in your state.
If you get beyond the SF-97/title issue, and you are unable to find another member to go, here would be my list of check items, maybe you could print and take as a checklist. Others here will have good ideas as well. This is mostly make/break, not nit-pick stuff that can be fixed, and this is probably like any other regular vehicle:
Does it seem to start and run well? Minimal exhaust smoke when cold/warm?
Any strange noises which would seem irregular to a diesel engine (hard metal knocking, something loose, vibrating, etc...)?
All gauges work and have proper readings (oil pressure may read low when warm, this is usually just poor gauge resolution in the low pressure range area)?
Does air build to 100-120psi range and hold (low air warning buzzer and light until 60psi)?
Transmission operate properly in all gears (torque converter locks up in 2nd, so it feels like it "shifts" even when held in 2nd)?
No water/coolant in engine or transmission oil?
Brakes work well? They are typically very strong on these trucks, especially when empty load.
No excessive engine/transmission/t-case/axle oil leak areas (wet)?
Vehicle should be able to accelerate to and hold 65-67mph (2100rpm) on flat ground as a check for full engine power and good transmission. 8.3L turbo will have delayed stop light take off until turbo wakes up.
Should drive reasonably straight, level, and vibration free except for tire tread noise/vibration.
Radiator look dry at core/bottom and tank ends/bottom (coolant full in reservoir tank, no air bubbles when running)?
Minimal to zero engine blow-by out of road draft tube when engine cold and warm? I have seen several 8.3L's have bad blow-by from excessive sitting, rings stick, then break on next start.
Minimal cab rot, check under passenger seat area from battery acid leaks.
Transfer case should give an audible "clunk" when engaged and dash light illuminate.
Known problem areas, mostly due to age, are: torque rod (control arm) end link bushings, fuel tank dip tube (suction straw inside tank), leaking soft fuel suction lines from tank to filter/engine, axle pinion seals, horn, PCB, batteries (suggest to keep battery age < 5 years old, can use $100 modern semi-truck batteries as economical replacement and space saver), and difficulty shifting transfer case in and out of low range (sometimes have to blow off all truck air pressure).
Smaller items which can be fixed are CTIS & ABS fault codes, engine fuel shut off solenoid (if 8.3L), burned out lights, worn tires, etc.....
Let the owner shut the engine off each and every time, as there is a specific switch order as to not over-volt the electrical system, unless an aftermarket civilian alternator has been installed.
