Any brand will work if you remove the stock head unit. Many people are going with Alpine, and others are using Kenwood. While it wasn't installed in a Mercedes, one of the best DVD's I've seen was a Clarion. Sorry, I don't remember the model.
One thing you might consider is retrofitting a COMAND. With the proper inputs it will display video from a dvd changer. You will have the benefit of a factory appearance, and if purchased from some sources a one year warranty on the COMAND.
I have the Kenwood installed. Well, the buttons on the steering wheel wouldn't work anymore just in case you don't know. I think the Kenwood headunit fits nicely in the car cause when the monitor flips out, it pretty much matches the interior of the car(with the silver and brushed aluminium look of the headunit).
Here's some pictures of my car if you're interested... http://www.csportcoupe.com/forums/showthread.php?s=&threadid=3071
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