Welcome to MercedesShop! You'll soon come to understand the addiction we all have to this site- for the great information, resources, and of course, all the great people here. To answer your questions (more or less), here's some recommended reading on the subject of W124 stereo upgrades:
This is by far the best link, as it is a fairly exhaustive description of one members quest for the best sound:
http://www.peachparts.com/shopforum/showthread.php?threadid=3149
Another bit of good 124-series reading:
http://www.peachparts.com/shopforum/showthread.php?threadid=3020
And, another good read, with other solutions:
http://www.peachparts.com/shopforum/showthread.php?threadid=3342
Welcome aboard, and enjoy your ride!
-Dr. Julio Fuentes, BS MD PhD