That's a real loaded question. Could be very little to a huge amount. Has it ever been updated from R12? How old are the hoses? I would not touch that with a very long pole
Well then the answer is "easy." Determine a fair price for the car based on maximum probable AC repairs. If the hoses, pump, etc LOOK like they've never been touched, then they probably haven't. New compressor and hoses, flush system, convert to R134A at the very least. If you have to pay for all that probly a grand off the top.
Well he's only wanting 2500. Needs master cylinder and brake lines. Haven't got to see it in person yet and he's really slow on returning any information
I really don't mind not having air, personally though.