Maheath had some good points - easy stuff first. The plugs in your car really need to be changed about every 15,000 miles. Use Bosch H9DCO. You may also use H9DC; however, the H9DCO has a slightly wider side electrode than the H9DC.
Mercedes calls for a .032 gap. Some folks here have reported that a gap of .040 provides a smoother idle. I settled for something in the middle -> .036.
I had a slightly rough idle awhile back and found the rotor to be dirty. I merely cleaned/smoothed it out with emory cloth. Did this on a weekend - parts shops were closed - smooth as glass after that.
You do not need to replace the entire plug wire assembly, just the resistors(piece that connects to spark plug). They screw off and back on to the wire fairly easily. A scope used by techs can help pin-point weak cyls.
It might be worth an hr. or two of shop labor to have the engine scoped and the fuel system analyzed. This way you don't waste a ton of money throwing parts at it.