IMHO rust is the deciding factor. Get under the car with a screwdriver and start poking. Pull off the body cladding. Pull up the carpet. Fell up on the truck roof, where the rear window gasket is. If it's really rust free keep it going. If it's rotten find a better car
I'd think oil fouling the plug is more of a valve guide or piston ring issue. A compression test would be a good place to start.
Can the valve guides be replaced with the head installed? I'd think the valve stem seals could be at least.
-J