Assuming the body and interior are in expected condition for the low mileage the "as is" asking price is reasonable. If everything worked and the car is clean the retail price would be about $7900 according to NADA, plus maybe a little premium for the mileage. The flex disks are a common wear item and, if the they are original, certainly would need replacement at the current mileage. They are usually crack by about 60K miles.
The older car A/C problems are not unique to Mercedes, but apply to almost any nearly 15 year old car, even with low mileage. These cars have very solid engines and drivetrains, but it's the accessories and gadgets that cause problems as these and most cars get older.
Even if you have to put two grand into it, it's probably a decent value, and listen carefully to Larry Bible's advice on repair and service of the A/C system. Don't skimp or take shortcuts. Get it done right, including going back to R-12, and a reputable independent A/C shop should be able to fix it as well as an average Mercedes dealer. The important thing is to contract with a shop that has a tech with W126 A/C experience.
For about the same money you could buy a five or six year old Camry or Accord with everything working, but it's not a Merc!