Cut and dry $500.
Here's why, plastic cladding can hide a lot, already having surface rust, would make me wonder what's lurking beneath, not to mention has it ever been painted and why. Car has been sitting for 15 years, you can bet on replacing the brake system, including hoses, calipers, master cylinder, wheel cylinders, pads & rotors, shoes and drums may be ok? Was it stored with gas? chances are the tank could be completely varnished and rusted on the inside, sender could be a rust ball as well. Clutch hydro system will likely need attention, maybe need to go as far as replacing the clutch itself. All your vacuum lines will likely need attention, as well as coolant hoses. I'd find out why it was parked in the first place, ie: what was wrong then. Carb will need to be rebuilt end of story, maybe a fuel pump. exhaust probably will need work before too long. Just because it ran then, doesn't mean it will come back to life and run perfect now (doesn't mean it won't either).
My point is not a negative one though, buying a car like this is a gamble, I took it, as others have too. The SX/4 I fixed up for my gf sat dormant for 15 years (parked due to a misdiagnosed clutch issue), everything I listed above is what I had to do to it, among other things I didn't mention (body, paint, cv's, axleshaft, bearings, water pump, valvecover, etc, etc, etc). My gamble paid off, the engine and trans (also 4 speed car) work beautifully now. Many others have had just as good of luck, but others haven't. Go over it very closely is the main thing, it could be a gem that needs very little.