Broken engine mounts: The engine will shake your entire car. Diesel engines have a lot of torque, and these mounts break often.
This W 123 crash test shows...
This W 123 crash test shows how well even older Mercedes-Benz cars are built.
Broken air-filter bushings: If the intake is loud and shaky and the air-filter housing is loose and bouncing around, it's easy to replace these cheap bushings. If left unattended, they can pull the turbo air intake off, allowing dirt and debris to enter the engine.
Car won't shut off: This problem is so common that Mercedes diesels have a shutoff button on the engine. This is most often a vacuum issue. Look for loose vacuum hoses and cracked vacuum lines. The vacuum controls the door locks, engine shutoff, transmission shift, brakes, climate controls, plus a host of other functions. Something as simple as a leaking door-lock valve can cause rough shifts and prevent your engine from shutting off.
Incorrect fuel gauge: This is caused by diesel sludge buildup on the tank sender, and it most likely just needs cleaning.
 Unlike the coupe, the 300TD...  Unlike the coupe, the 300TD station wagon is often priced out of the cheap range. No wonder-they're turbocharged, get good mileage, and have lots of cargo space. |  The W 124-series was built...  The W 124-series was built in three body styles, but only the sedan and wagon came with a diesel engine (as far as we can tell). |  This setup became known as...  This setup became known as the Whisper Diesel because of the numerous sound-deadening panels and a completely encapsulated engine. |
Fuses: The fuse holders are copper, while the fuse is often aluminum. The result is corrosion and a good fuse that isn't making contact. You can test both sides of the fuse with a test light or, in an emergency, wiggle the fuse to make contact. It's best to use copper replacement fuses.
You can sink a fortune into these cars and always have something new that you can fix. Or you can sink in next to nothing, and your car will still last forever. I suggest bringing the car to the condition that you're comfortable with, relax, and enjoy years of reliable service.
-Brian Friedman, www.lovecraftbiofuels.com