Date registered: Dec 2005
Vehicle: 2005 C350, 2002 ML270CDI, 1969 230S, previously 2002 A160, 1985 380SEC ivory, 1983 380SEC grey
Location: Adelaide South Australia
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the most important is lack of rust in the body! All old cars collect rust - and it can be VERY expensive to repair. The W114 or W115 bodies can have bad rust on the driver's floor and fire wall, meaning that the first big bump you drive over results in a broken body (slight exageration, but not much). The coupes also rust badly in the side rails under the doors. In Australia I have seen examples where you can use your finger to poke holes in the metal.
So in summary, look for minimal rust (the cars are 35 to 40 years old, after all), and a good provenance and service history. Regular oil changes are good, regular coolant chnages even better, as the engines have an aluminium cylinder head and cast iron engine block. The use of water in the radiator starts a corrosion process which is "quite expensive" to fix.
I have recently bought a 1968 W114 230 Automatic with a blown head gasket. The head was not useable, as it had previously cracked and been welded, and the aluminium was softened as a result, so even if welded again, it would be weak and liable to failure. Eventually I found a source who sold me a new cylinder head from an engine which had been imported from Europe, so it had always had proper coolant (for their winters) and so had almost no internal corrosion. That was a mere AUD500, and another AUD2900 in parts and labour got the engine working.
All in all considerably more than the car is worth! However, this car did have its original books and the 1968 delivery docket form the local dealer, so if I keep it a few more years, maybe it will appreciate. Hmmmm.
Anyway, all the best in your search
regards from sunny South Australia