My suggestions would have to be:
Check to make sure the lift pump is working. Bump the key and listen for it. If it's quit, it may have taken the VP with it.
Look for oil leaks on the engine, rears, transmission.
Look to see if the owner has hacked on the wire loom anywhere. I hate having to fix rat nests!!
Broken dash, seat covers, trailer hitches, etc. Any welded on hitches or accessories, check the welds/attaching plates for cracks/bends/signs of damage.
Look at the overloads, if equipped. They'll tell you if he's been hauling heavy with it if the pads are worn or polished from being loaded heavy.
The list goes on... ... . One big thing I look for are signs of a programmer, like velcro strips on the fuse box cover, wire taps on the injection pump wires, guages, mystery switches and wires, etc. I'd also suggest looking under it at the skid plate and under body panels to see if it's been driven over any rocks, brush, trees, small boulders, large animals, or small to midsized foreign cars. Also ask if he has any service logs... if he has a record of every oil change, tire rotation and light bulb change, chances are he took good care of it, and it's a good buy. Good luck!!