Having had a 1993 300e 3.2 and currently driving a 1994 e420, I can speak a bit on this subject from experience. When I bought my 1993 about 10 years ago, I was not part of this forum. Had I been, I would have known to check for a leaky head gasket, bad evaporator and a decomposing wire harness. Unfortunately, I had to deal with the evaporator and head gasket on my watch. I believe the leaky head gasket was a problem on the 1992 and later m104 (3.2 liter) motors (I don't know if the smaller 2.8 liter ones had similar issues). The evaporator problem, I believe also primarily occurred on 1992 and later cars, as well as the decomposing wire harness caused by the use of biodegradable insulation. Replacing the evaporator requires the entire dashboard to be removed. On the upper wire harness, there is usually a tag on the harness itself near the firewall where the date can be checked. Also, the lower wire harness and throttle body harness were apparently affected as well on 1992 and later cars.

I have had my e420 for a little over a year and have only replaced the camshaft oil tubes and upper timing chain rails (about $400 in parts and I did the work myself), which is a known problem on the V-8 cars. Finally, the youngest of these cars is going on 20 years old, I would thoroughly check all rubber parts (ball joints, bushings, tie rods, center link) on the complex multi-link suspension. On my e420, I had to replace the tie rods, center link and sway bar bushings (these are an easy fix). Also, apparently the flex discs also wear but I have not experienced this problem yet (knock on wood). This goes for all other rubber parts as well (e.g. motor mounts and hoses).

I would also check the exhaust for leaks. I had to replace the exhaust in the e420 from the catalytic converters back. On my car, the exhaust was extremely expensive if I bought new (e.g., $1500 in parts) but I lucked out and found a very nice used oem exhaust. I think there are more aftermarket exhaust options for the 6 cylinder cars.

Check to make sure the mono wiper works and is not slow or gets stuck. I forgot to test this on my e420 and, of course, it didn't work right! The wiper is supposed to be lubricated regularly but lots of owners probably forget to do this. Sometimes cleaning and lubricating works, sometimes the gears are worn and a new wiper transmission must be installed (these are still readily available from parts cars but is impossibly expensive from the dealer), or the motor is worn (less expensive).

One thing I am amazed at MB's rustproofing on these cars to the extent that my cars looked much better than earlier w123's or even later w210 cars. I would, however, still check each of the jack points and inside the trunk wells (caused by water leaks) for rust.

I have not owned the earlier w124's, but a friend of mine had a 1990 260e. It was a fantastic car but suffered from excessive oil consumption due to leaky valve stem seals. He maintained his car well but drove it hard.

I am sure that there are things that I missed....

__________________
Brian
'94 E420 - 99K
'12 C300 4 Matic

Previously owned:
'06 CLK 500K Cabriolet
'85 300SD (Old Smokey)
2002 Boxster
2002 C320 (The five-pointed star Mercedes)
1993 300E 3.2 (Dealership's cash cow)
1979 300SD (Chrome beauty)
1983 300D (Old faithful)
1983 240D (Right lane cruiser)
1977 240D (Miracle of bondo)