ok - first thing first. here
watch his series on youtube. there are 5 in total on buying trucks. the intro - which i posted. and parts 1-4. some people on here dont like him, but i think he makes great videos and personally he has helped me a few times
under the truck - the very first things to rust and rot out are usually the factory molded abs running board steps - which they do NOT make anymore (if the truck is so equipped) and the rear leaf spring hangers and shackles. factory sets are riverted on, so if there are any bolts, that means they have been replaced. if the factory hangers are there, intact with no rust,rot or holes - then chances are the rest of the truck is pretty good as far as rust.
check all fluid levels obviously, and see if the owner will allow you to remove the intake to the turbo. check turbo for any shaft play as that will be an early warning sign of failure.
if truck has dual tanks, check to see if the level indocator works for both tanks. ive had 2 of these trucks and both had one that didnt work. also, check the rear fuel tank and straps for rust. alog with the leaf spring hangers, the rear tank is also another spot to develop problems.
obviously check the lights on the dash. make sure the wait to start light cycles on and off. make sure the struck strarts up easy and doesnt blow any smoke. some white smoke upon a cold start is ok, but it should not be blue. make sure the check engine light in not on.
make sure it runs and drives smooth. if you can, jack up front of truck and check ball joints for play. also - it is not uncommon for the manual locking hubs to get stuck engagaed, which usually just needs a good cleaning. one oil leak to be sure to check for, is the dipstick where it enters the oil pan. that one leak can be the source of a real headache, and would def be something you would rather NOT need to address.
make sure to watch the series on youtube. he does a great job..