They’re great machines, and unlike the later cars you don’t have to worry about having to hook up an OBD2 unit to diagnosis problems/reset the warning lights - the systems were still mostly mechanical. That said, if you buy one of these cars you’re going to have predictable problems.

• Don’t put down the rear windows. Seriously. They’re run by a garbage pulley system with support pieces made of plastic, and they break; your window will go down and not go back up. The rear window regulator, as it’s called, is a $130 part, and you have to take the door apart to install it. I wrote Mercedes a letter.
• The engine wiring harness is basically biodegradable. It’s the system of interconnects that handles all the electrical systems of the car, and its plastic insulation wears down, exposing bare wires, and once they start touching, you begin to have weird problems - the A/C fans won’t turn off, roughness with acceleration and so on. The harness costs $550, and it can legitimately take two hours to install, so if you have the work done at Mercedes-Benz expect to pay around $1000.
• The vacuum systems. There are pneumatic systems that suck the doors shut when they’re not fully closed, pneumatics that control the door locks (classier than magnets, but troublesome over time), and a few others. They only work when a vacuum is acheived, so a pinpoint hole or small rupture can mean having to replace a line or pump.
• The master brake cylinder goes. You notice there’s too much pedal travel when you press the brake; you get new pads and rotors, and the problem persists. You then have to replace the master cylinder, which costs $500. A huge price for this kind of part, and another $1k repair if you go to MB.

Those are the main problems I’ve heard of; I’ve had the window issue myself and anticipate the others as time passes. Also, I’m not a mechanic, but if you want to avoid being ripped off when it comes to car repair, you have to learn a little bit yourself. My advice is: any used 90’s Mercedes sedan you buy, put aside $4000 above the purchase price in a fund for three years of maintenance.