An AC evaporator is about $400. Factor in the cost of the items you usually replace when you service the refiregerant system like the dryer and expansion valve. Then there's purging and charging the system. It takes an experienced hand something like 20 hours to take the dash apart to replace the AC evaporator. I don't know if special tools are involved. Unless your father has replaced the AC evaporator on a W140, don't get a fixer upper.

The problems with early W140s are shared by all W140s. The basic advice is to get the latest model W140 you can afford. Hopefully you'll find one with Starmark warranty. Experience fixing cars, even the S-class that preceded the W140 will count for very little when dealing with a W140. The systems that go bad are systems a reasonable mind wouldn't expect a car to have.

Sixto
95 S420
87 300SDL