I bought my 1987 190E, champagne metallic exterior/burgundy interior, 10 years ago from the first owner with 125K kilometers.
The car was in very good shape, just had to do some routine maintenance (full tune up, auto transmission fluid and filter, front suspension tie rods, differential oil, etc.) and change the front plastic vent grills from the dash, which breaks easily and often.
After ten years and 100K kilometers, I have spent around $3000 just in parts including tires, batteries and oil changes, and I do not regret it at all. Routine maintenance parts are very cheap, even at the dealer. Major repairs:
Radiator (at the dealer, $250), changed it myself along with genuine MB coolant (highly recommended).
Water pump (twice @ $70 each) and hoses.
Belt tensioner once @ $100, very easy to change.
Gas electric pump and relay once, @ $100 each, again easy to change but a little messy.
A/C evaporator @ $200 (just for the part). Not difficult but done by specialist.
Transmission flex disc @ $70. Easy job but a little time consuming.
Brakes (rotors, pads, fluid), cheap and easy to change.
Wiper mechanism fully dismantled for routine lubrication. Not that difficult, a little time consuming. Beware not to break the windshield!
Valves seals once. Parts costs $15, and changed myself with new tool, patience and Haynes repair book.
A couple of suspension arms in the rear @ $20 each.
New alternator bearings, new starter solenoid.
A lot of other minor parts (fuel injectors, hoses, some electric niggles, etc.). Needs new motor mounts.
For a do-it-yourself-er as me, this is a dream car to work around. German engineering at its best. Parts last long, are cheap, available and easy to take apart for cleaning or replace.
This car, even after 23 years and 225K kilometers, is a delight to drive, for short or long trips. It has great handling, exceptional brakes, excellent visibility all around, strong but comfortable suspension.
On the not-so-good department, this is an old car, and as any old car some parts are reaching their life time (starter, generator, windows regulators, A/C compressor, switches, etc) and might need to be changed at some point. Not the car's fault, though.
Some plastics or trim pieces are fragile (dash, console). The cabin is small, especially in the rear, and has a poor sound system design (before 1989). The cooling system of 4 cylinder engines might give trouble, and finally, the door panels. They were poorly designed, and after some years and heat the vinyl/fabric trend to shrink and separate from the panel itself beyond repair. New panels from dealer are available (as any part) for $300 each, but for a $2000 car, it does not make sense. So expect to spend some $$$, even more if you know nothing about mechanics.
The engines and transmissions are bulletproof, but some externals might give some trouble. If the vacuum system is leaking, the engine might run poorly and transmission might shift hard, misleading the diagnosis. Do not underestimate this (I learned the hard way).
Advice? If you are considering buying one, try to find a post lift model (89 and up) since they were substantially improved (seats, stereo, exterior plastic moldings, etc.). Consider a 2.3 engine (same gas figures as the 2.0 engine, but more power), or even better the 2.6 engine (more thirsty but more power, requires less maintenance and makes an excellent marriage with the auto transmission).
And finally: there is nothing more expensive than a cheap Mercedes. Pay a little extra for a good one, and do not forget that any Mercedes requires proactive maintenance.