Over the last 10 years and with with synthetic oil becoming a main stream product, service intervals have certainly changed. Many posts and articles that I read advocate extending the oil change intervals upward to 20k miles.
However, every different make and model, as well as each individual's driving habits, affect service intervals, and I would rather err on the side of caution as opposed to some variable dictated by monitors and oil tests, possibly conducted on some newer MB models. The Benz Store's recommendation for oil change intervals for model 2000 and newer:
under 125,000 miles: 7,500 – 10,000 miles
over 125,000 miles: every 7,500 miles
Most of the expensive repairs that we do are transmission related. Most people will religiously change their oil, thinking that they are servicing their cars.
Personally, I would recommend changing transmission fluid just as often as engine oil but I know this gets expensive, especially on the post 2007 cars. Transmission services need to be performed when the car is cold and the tech must have access to the Star diagnostic system.
Replacing a transmission on the 2007 and newer cars is an expensive ($5000) financial nightmare. So it is critical that transmissions be serviced. Intervals should not exceed 35,000 miles. I know this suggests overkill, but I cannot stress this more emphatically. This service costs roughly $300, but if you own one of these cars with a 722.9 transmission (2006) and newer you have to do this!
On the 2006 Mercedes and older, I would change fluids every 30,000 miles. In between these 30,000 mile services, at 15,000 miles, fluid only would be changed. I know there are some individuals who are rolling their eyes, but in my immediate family there are currently 3 cars with an excess of 300,000 miles, each with the original transmission and engine, and they last because of religious fluid and filter changes. In 35 years of owning various Mercedes, I have never had a transmission or engine failure under 300,000 miles.
Also, coolant should be changed every 2 years. This is so important on older cars because they have developed leaks and people will refill with just water, creating an imbalance in the 50/50 mixture. This will affect everything including the water pump, radiator, thermostat, hoses, etc. The estimated cost is $100.
Brake fluid should be changed every 2 years, estimated cost is $100.
Change the air and cabin filter every 12,000 miles, the fuel filter every 50,000 miles.
The high end models, such as the S, SL, CL, CLS, do have systems (airmatic, ABC-active body control, top operating systems, closing assist, pneumatic locking, and others, which have no preventive maintenance. However, if you want one of the above-mentioned cars, the cost of ownership can be mitigated by due diligence. If possible, buy from a first generation owner, who bought, not leased. Get all previous repair orders – obviously you want a nice dossier of repairs. And finally, get a comprehensive pre-purchase inspection. Finding a car that meets these criteria will be difficult, but you can reduce your time and effort by talking to the individual before you visit the prospective purchase. Be specific in your questions concerning service and repairs. Finally, find a good independent mechanic and put your trust in him. Trust this mechanic until he (or she) gives you a reason not to trust. A good technician will explain to you in detail what he is going to do.