You'll want to know what kind of regular maintenance was done and at what intervals. You'll want to ask about the injectors (there's a good chance they were replaced/rebuilt already so you'll want to know when they were done), has there been any trans work and has that fluid been changed ever. Any front end work (hubs, suspension, etc)? Plan on changing all the fluids in the truck once you get it just so you know they have been done. This also will give an indication of the health of the different components.
Those are the questions you could ask, but if the body is good and the truck runs, it's probably worth $3500 regardless of any of that info I just suggested.
In most cases the body will fall away before the motor and trans give out if they're taken care of reasonably well. These motors go forever. One real quick thing to look at is whether or not the back tires have considerable less tread than the front. It will tell you if the owner has been unkind to the truck on a regular basis. Other than that go for a drive with it and see if it feels right. You'll know if something isn't right. Look for excessive blue or black smoke from the exhaust. It will probably puff a bit on startup, but should be pretty clear while driving. If you stomp on it, it will puff black smoke while the turbo spools up and that's normal.