Engine: How many miles? Idle quality, power levels.
Transmission: Firm nice shifts, clean looking/smelling fluid. (reddish, should not look gray or brown)
When driving listen for creaking in the suspension, could be worn bushings or joints.

Check the A/C, heat, and all power accessories. Look for oil leaks on the engine.
Check exhaust system. Look for rust...depending where you live.

W124 is a solid beast, not very hard to maintain a 300E. Most troublesome items are usually the A/C and the idle control on the engine.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)