Here is some more info I recieved from a Mercedes mechanic regarding a1990 300ce I am (was?) considering buying.

Mercedes-Benz 300CE Problems 1990ish


Brakes

• Due to the size and materials of the brakes on these vehicles, the brakes start to squeak around the 50 percent wear point. The brake rotor surfaces become uneven, causing a lip to form at the outer edge of the rotor. Because of this, the rotors are usually replaced when the pads are worn (pad life varies depending on driving style and terrain).
• Brake fluid should be flushed every two years to prevent premature anti-lock brake system (ABS) failure.

Engine

• Oil leaks develop from the head gasket and timing cover gasket; these leaks should be repaired before the engine oil and coolant intermix.
• The part that attaches the distributor rotor to the camshaft can break; the starter will still crank, but the engine will not start.(change this with the first tuneup there is an updated part for it that should prevent further problems)
• Radiators break due to degradation of the plastic tanks and fittings.
• Older vehicles should have the cooling system serviced even if the mileage is low. The hoses can look good on the outside, but the rubber can degrade and erode on the inside. If not carefully checked, the hoses can burst at any time.
• The water pump can develop leaks. When replacing the water pump, the drive belt tensioner should be inspected and most likely replaced as well.
• Motor mounts fail, causing engine vibrations to transfer to the body. The motor mounts should be replaced or the transmission mount may fail. (makes a big diff in how smooth the engine feels)

Exhaust & Emissions

• A rough running motor or lack of power can be caused by the distributor cap and rotor, bad ignition wires (or their resistors), or valve cover gasket leaks. When the valve cover gasket leaks, oil can pool where the ignition wire resistor connects to the spark plug. The resistor can fail or the spark plug not fire because of the short circuit through the oil.
• Fuel leaks, erratic or rough engine operation, and fuel economy issues can be caused by the failure of a fuel system component called the EHA (electro-hydraulic actuator) valve.
• Emissions air injection pumps can seize up, causing a loud squealing noise possibly followed by smoke. The air pump pulley stops turning, but the drive belt does not and this friction causes the noise and smoke. If the engine is not turned off, the belt will snap, turning on the alternator/battery warning light.

Suspension & Steering

• Irregular and premature tire wear can be caused by the lower control arm bushings cracking and causing excessive movement at the control arm pivot point. Ball joints can also wear out prematurely, causing similar tire wear symptoms.
• The tie rod ends, drag link, and idler arm in the linkage of the steering system can wear out.
• There is a steering dampening shock that will leak fluid when it goes bad.
• A knocking noise from the front end can be caused by cracked strut tower bushings.
• Vehicles with level control that ride harshly in the rear (no give in the suspension, like you are riding on a brick) may have bad dampening actuators. The actuators should be replaced in pairs.

Drive Train

• A vibration felt through the center floor of the vehicle can be caused by driveshaft flex discs, which crack and shred, causing excessive driveshaft movement. Not repairing these can cause damage to the driveshaft center support bearing, or, in the worst case scenario, damage to the transmission or differential. (check this, its a lot cheaper to catch this early)
• High mileage vehicles can leak from the transmission oil seals.
• High mileage or older vehicles develop leaks at the differential seals and cover; the whole unit needs to be resealed to repair the leaks.
• The bushings for the shifter lever wear out to the point where they break and fall out. This causes excessive movement (loose feel) in the shifter lever and a clanging-type noise when changing gears. (cheap & easy)

Electrical & Lights

• The insulation degrades on the engine wiring harnesses because the insulating material falls off of the wires and leaves the conductors exposed. This can cause many problems depending on which wires are exposed and what they touch.
• Throttle valves can suffer electrical failure.
• The battery can leak acid onto the engine compartment wiring harness, causing damage to the wiring from corrosion. (check this before purchase, you dont want to repair this)

Heating & Air Conditioning

• Due to wear of mechanical parts in the control head, the buttons on temperature control assemblies work erratically or not at all. These control heads need to be replaced for this concern or when the display screen starts to fade out.
• Due to the vacuum actuators going bad, temperature blend doors in the HVAC system may stop working.

On top of that list I suggest:
- Changing the fuel injector seals (if original they will be leaking vacuum by now)
- While the injectors are out have them tested for flow pattern and leakdown (if they have never been done they are almost guaranteed to have a fault).
- Check the front fuse box (under hood near hinge on drivers side) for signs of water infiltration, might cause intermittent electrical problems in future.
- Those and O2 sensor and other vacuum leaks are the main problems for poor fuel economy & smooth engine running.