The 2.6L six is a smooth powerplant compared to the 2.3L four; however the M103 motor does have a known tendency to have leaky headgaskets past 50-60K miles or more. The engine compartment is pretty tight with the longer engine, making any kind of work up front (belts, timing chaing, water pump) an exercise in draining and removing the radiator and supports.

The 2.6L was Mercedes' response to the BMW 5-Series in the US showroom wars. You may also want to consider the W124 300E from the same time period and certainly the 3.0L twincam version.