Had the transmission rebuilt when I bought the car ($1400 CDN).
Minor surface rust repair (parking lot dings).
Some interior trim put back together.
Not much else it needs at this time (I don't consider routine maintenance to be a repair).
And yes, the power windows all work (a problem with any M-B).
I bought this car last year for $1500 CDN, as it had a sick transmission. This particular transmission, also used in certain other M-B models had a problem with a servo piston hanging up or failing outright, causing no forward gears or severe slippage, though reverse worked normally. Having the transmission rebuilt completely was insurance that it would continue to soldier on, though my transmissions' problem was in fact the servo piston.
That being sorted, I am very happy with this car. I have driven many Mercedes before, but this is my first time owning one. The turbo-diesel makes for decent power over non-turbo engines, (not quite like my V8 Chevy though - see my article on my white on green '78 Chevy Caprice 4-door, which I still have:) ) and with way better economy than any gasoline engine, plus far more long-lived. The engine starts right away, even when cold, and still has excellent compression, and oil consumption is negligible.
The car handles exceptionally well on the highway; it seems to become more stable the faster I go. In town, it is well behaved, and is easy to park for its size; it has a smaller turning circle than other cars it's size. The cabin is roomy, and comfortable. It has supportive seats that are very conducive to the drivers' needs; however I have noticed that some of the interior trim could have been designed a bit more carefully, in particular the plastic around the back window, on my car; once off, it doesn't want to go back on easily. The controls take some getting used to, but are quite logical once you figure them out. The car still has it's Becker cassette stereo, which is a bit confusing to use at first, but like the rest of the car, it has a logic of operation all it's own. The heater and air-conditioning works well enough, once you get it set up (mine is the automatic type).
I like the styling of the W126's, modern, but with still a bit of the older M-B styling (I find the W140's to be somewhat coldly forbidding, but still are good looking). Nice wood on the dash, chrome in all the right places, high quality paintwork (white with tan M-B tex interior) which, unfortunately, needs some attention due to inconsiderate drivers/passengers of other cars in parking lots. No accident damage though.
In my opinion, it is screwed together better than any other car out there, and is actually not too hard to work on (I do my own mechanical work). I know a fellow with an '82 M-B turbo-diesel (300D, W123) with 495,000 KM on it, which I find reassuring, knowing that with proper maintenance, my car will have lots of life in it yet. I think I'll hang on to this one... :)