Welcome! Doing the flush was one of my 1st tasks when I acquired my 300D. The engine is the same so this info should apply to you also, but not guaranteed.
Check out my post listed below under my name which tells how I learned where the block drain is and the flush procedure as taught to me by a MB tech. I found the block drain by following the oil drain tube under the air filter housing.... you can't see it from above but you can feel it. Am not sure on the SD though... I think you have more room. If you do this procedure I might suggest changing the thermostat at the same time. This is an inexpensive part but can be very important this time of year. Also, be very careful not to lean on the radiator overflow tube that comes out of the top of the radiator. I broke mine this way and although fixable is a royal pita.
http://www.peachparts.com/shopforum/showthread.php?threadid=74295
A steam clean or gunk clean is also helpful 1st to keep from getting too filthy.
My block drain was in very tight... may never have been removed before.
Let me know if this helps