My Top 5:
1. "Killer Dowel Pin" (KDP). Plenty of posts on this site to read on this topic. Try to find out if it has already been replaced or not. If not its no big deal... it's an easy fix (TST sells a nice fix kit)... but you'd like to know whether or not its already been done before you go tearing into your engine to fix it. If it hasn't yet been fixed, DO SO IMMEDIATELY AFTER BUYING THE TRUCK; if it fails on you, you'll be really sorry.
2. 'Quick disconnects' on the transmission cooling lines (for an auto trans). I don't know what year Dodge stopped using these POS connectors (my '95 had 'em). If yours has 'em you'll want to replace them; they leak, and they fail catastrophically.
3. Fuel lines. After a decade or so the rubber fuel lines will develop leaks. Its nice if yours have already been replaced. If not, it's just another job will need doin' someday.
4. Look under the batteries (particularly drivers' side) for corrosion due to battery leakage. Then replace those nasty old batteries with something what don't leak, like Optimas.
5. Death rattle. While test-driving a vehicle, try hitting as many potholes and railroad tracks as you can, at 50+ MPH, at different angles. If the truck feels like it will go out of control, pass on the deal. Making a death rattle go away is an art, not a science, and there are no guarantees.