Call a few local shops and see what they quote. Not knowing the source of the leak (leaking condenser, bad compressor or if it's just a cheap fix but has been dry for a while and needs a new accumulator) there could be a range of prices. An evaporator leak would be nearly a deal-breaker for me since you have to pull the dash to get to it.
For example, a shop charged me $70 for a vac down and recharge.
New hoses to the condenser/accumulator plus new orifice tube and accumulator ran me $500 this summer (2-door, 98).
A compressor or condenser replacement is maybe an hour, hour and a half each in book time. Figure $80 an hour for labor, maybe $100/hr at a dealer.
You could probably add up the retail cost of parts, double it and you'd probably be on the low end of what a shop would charge you for parts and labor.
I'd kind of doubt that it needs a completely new system. It may be worth taking it to a shop, eating an hour of labor and have them do a leak test so you'll know what you're looking at.
A/C is one of those things that I'll pay a shop to do right the first time.
-end rambling-
Good luck with the purchase.
Oh, check for coolant in the oil and, if you can spot it, any coolant leaks where the block meets the lower intake. Ask seller if the lower intake gaskets have been replaced yet.
