Going off memory since I was not working on those trucks by then (stopped pretending I was a mechanic around 07)
Things to watch for, power steering, the older 1500s relied more on the pump and pinion rather than mechanical advantage, so that can cause problems.
Seals around diffs and transfer-case. As well as maintenance, if the diffs were left a long time between services, they may suffer, depends on if its got limited slip or not.
500$ seems low for the truck, but that's also double its expected mileage for the age. Long highway are the easiest miles, unless it was towing the entire time.
If he's willing to gamble on having a few thousand in repairs, then it might be worth while, but I'm going to guess that its going to be about done in at this point unless the owner was fanatical about maintenance. It'll probably be a water dripping, oil burning, cash chewing beast. I'd be thinking that if its on the original drivetrain, the transmission and engine may not have much left, if by chance its a manual, then a test drive will show if any gears are worn, and most age can be fixed with a new clutch. If its an auto, then a rebuild or a low mileage used from a wreak should be easy enough to find.
At this age aftermarket is going to be as good and possibly cheaper than MOPAR, and apart from electronics, they are reasonably easy to work on (provided you have the tools and space, hoist)
In any case the truck won't end up costing 500$, but if he's happy with that, he could be getting a lot of tuck for a few grand.
