Hamish,

I have an 86' 300E, witch i have purchased used so i don't have the exact history of the car. However since i have it, for about 18 months and 120.00 kilometers the only problem i had was with the engine coolant sensor ($70.00 canadian) and the ignition coil ($40.00 canadian used), other than that it runs just fine. It burns about 2 quarts of oil per 30000 miles (total milage is about 240.000) i am not sure if it is because of worn guides or rings. In any case it runs just fine. Not as fast as it should be but i still love it. The water pump was replaced at about 200.000 miles. Same as my friends 88' 300E. It cost about $400.00 canadian for a new one.
Other than that it runs fine. Good luck.
Alex