I've been maintaining my 98 E320 since I bought it 1 1/2 years ago. Since the transmission is a sealed unit and uses expensive synthetic trans fluid, it is not considered a DIY job. Mercedes claims that the fluid will last the life of the car, I will have my changed at 100K miles along with the spark plugs since I intend to keep the car "forever".
Mercedes has a special tool for checking the transmission fluid, but the tool may be difficult to obtain since the temperature of the fluid is critical for obtaining the proper level.
Mercedes recommends synthetic motor oil for the vehicles with FSS. I use mobil 1 0W-40 for the winter and mobil 1 15W-50 for the summer. The V6 uses 8.5 qts. of oil. The FSS suggests oil change intervals around 10,000 miles. Since I intend to keep my car "forever" I change the oil and filter every 3,000 miles which equates to twice a year based on the amount of miles I put on a year.
The owners manual explains on how to reset the FSS, and the oil filters are relatively inexpensive through fastlane. The gas and charcole filters are still expensive.
Maintenance that I did myself beside oil and filter changes includes rear differential fluid change, radiator fluid change, brake fluid flush, and gas filter.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles