Four grand is about right. After all, nearly everything that usually goes wrong has been fixed already!
For a year or so you can live with the window that won't work and the seat that is slow -- unless someone is adjusting the seat all the time, what's the problem? Pull the console switch for the rear window so it won't get rolled down.
The seat fix may just be a need for lubrication, and the rear window is probably just a need for adjustment, but the motor could be shot, and that gets expensive. Cheap if you can find a U-pull-it salvage yard, though.
The only other thing I would recommend is that you have another shop check the timing chain. Since the owners maintained it, this shouldn't be a problem, check and see when they had it replaced last. This is the real Achilles Heel on this car -- single row timing chain wears out too fast, snaps one of the guides off, and eats the valves. $4000 min. repair bill. Cure is to check stretch all the time and replace when needed.
Nice car for the bucks! A bit underpowered, but nice.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!