@BJ I can't speak to the rust issue, since mine s a west coast truck from the high desert, it has none.
Mechanically, they're solid; both the old Gen 2 small blocks and the later LS- based engines are bulletproof. They all get wretched mileage, so get a big motor. Avoid the 4.3 with an automatic; it's utterly gutless. Transmissions are mostly NV3500 if you get a manual, or a 4L60E for automatics, both pretty reliable. The automatics do wear out after a while, but they're a hell of a lot more durable than Ford equivalents.
Steering boxes and rag joints in the column can get loose, and as with any older GM product, plan on replacing the tie rod ends and ball joints at some point. GM seems to design them to last just about 100,000 miles, but parts are dirt cheap.
Interior plastics break, electrical stuff quits working, but the basic core of the truck will chug along regardless.