There are so many companies out there that specifically tune with Mercedes Benz only. Today it has become a highly competitive industry. Most of us have heard of AMG, Brabus and Carlsson.
I have been following a Danish company called Kleemann Mercedes. You can check out my blog at kleemann-mercedes.blogspot.com
Kleeman is a pretty old (and very well regarded) tuning company. Every MB enthusiast should have heard of them. Same with RennTech, VRP, Powerchip, MKB, Vath, etc. Most of those are performance primarily. For body kits, you have the aformentioned Brabus and Carlsson...Wald, Lorinser, Expression, etc