Future diesel posts - use the Diesel forum:
Diesel Discussion - PeachParts Mercedes ShopForum
Generally a rough idle is caused by incomplete combustion. If you get black smoke on heavy acceleration, you may simply have a lot of carbon build-up in the pre-chambers. Driving the car hard using quality clean fuel will help. Search the Diesel forum for "Italian Tune-up". There is also a product from Lubro-moly called "Diesel Purge", add a can to your next tank of fuel.
If none of these help, then perhaps you've got an injector issue. Service life seems to be about 100k miles. There is a heat shield under each injector which is one-time use so you need to replace these if you pull/replace the injectors. Rebuilt Bosch injectors are a great value, but the pressure spec for new injectors is a 10 bar range. In order to get a smooth idle, the injector pressures should all fall in a 5 bar range, and if you want a REALLY smooth idle, a 3 bar range.
There may be a Bosch service center near you that can test, clean, rebuild if necessary and set the pressures. Last time I did this, several years ago, they charged $25 per injector, took a couple days. Last time I purchased rebuilt injectors they were $75 each.