The strong odors from the exhaust means that the car is running rich. Running rich is kept in check with the O2 sensor and Mass Air Flow sensor plus many other components. Mass Airflow Sensor is probably shot and will need replacement as they lose their acuracy as time passes and they are positioned above stainless steel headers. Also, 02 sensor will need to be checked and or replaced. Both catalysts probably need to be changed and see if the 8 year warranty will cover this. First, see whether the warranty will cover this because you definitely dont want to be end up paying list prices for these cats. If the warranty doesnt cover this, take it to an MB indie. Running rich can be caused by many things but I just gave the scenario which happened to me which is the best case scenario. The dealer may be stumped by this one and will want to just throw parts at the car like $2k wiring harness which is probably causing your rough idle. Once my car had its cats changed, the service advisor said they billed Mercedes twenty hours. Low octane fuel or low quality fuel voids the emmisions warranty, by the way. I never had a check engine light, either. The CE light comes on with complete failure of a component not due to aging. Your plugs have only 40k miles on them and they wouldn't have caused this unless they failed terribly.