My M880 is now a terrific 10mpg Iowa farm truck. Its simple and easy to work on. Most chronic problems that I have encountered are shared by other 40 year old pickups, except these on mine are inexpensive to fix if you have the time, experience, tools, and patience.
Fuel tank, vents and lines should be removed and refurbished. Mine had about a gallon of water and some other unidentified liquid or tar that needed cleaning; the metal vent system was dissolving into rusty pieces that fell into the tank. This rusty tar had taken out fuel pumps before I got it.
The wiring harness had had several patches that needed to be cut out and repaired. The two most common wiring glitches in the M880 is well documented by others in this forum. 1) The
ammeter needs to be bypassed and 2) firewall bulkhead plugs should be carefully checked and patched through if necessary.
Condition of your prospect is everything. I have seen the bodies and sheet metal of some in much better shape than mine was. Good Luck.