I too just bought a 1985 500sel, and luckly for me it was well maintained (new trans, excellent body, engine had been serviced, compression was right on). Since I got it, I had a MB person do a front to rear check on it, and I had to do the following:
1: New water pump
2: New front wheel bearings
3: New hood liner
4: Replaced interior lights
5: Rotor button
6: Remove dust shield (only one, strangly enough) from front
7: Fix broken vacum line
8: Change transmission fluid
9: Fix rear seats heat and motors
All in all, with the inital checkup of the car, I've spent $1700. Now I don't have a car payment, the vehicle has over 200,000 miles on it, and I don't ever plan on getting rid of it, so I think it's cheap so far.
Of course, I didn't have to replace the timing chain, and I'm looking at a new rear main seal, new fan clutch and maybe a steering gear box sometime this summer. Ask me about money then!
Good luck if you get the car!