Porsche was doing well with their zinc plated bodies by the time 1983 came around. So, while these cars do rust, it takes a lot more abuse to get them to rust.
If this '83 needs front fenders, I would bet there is quite a bit more rust that you haven't seen yet and may not be able to see even with a good inspection. It takes a lot of exposure to salt/moisture to get these zinc plated cars to rust out.
It's very hard to put a value on this car without knowing more details. Does it run and drive? How well does it run and drive? Besides the A/C issue, what other major issues does this car have?
If this car is a basket case with extensive rust and in need of major repairs (clutch, brake/suspension work, trans work, engine work, interior freshening, paint and body work), it's not worth much. $2000-$5000 maybe... You could spend $10,000 'restoring' the car and still not be done... You could spend $20,000 on it and still be left with a car worth $11,000...
Unfortunately, these types of cars (at this point in time) are getting to be just parts cars. Restoring such things is either left to serious DIY guys that want a challenge and/or the car has large sentimental value.
Good Luck,
Jay
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