The most important thing is service records, whther it has been in an accidnt or not, and the issues others have mentioned.

Another very important item, is a trusted independent tech who can give youa through Pre purchase inspection.

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon - Sold
2006 E350 Wagon - 368 - Dead
2015 GL350 Bluetec - 197U