I had C63 back boxes installed on my previously owned E55K. Really brought the V8 noise out, and certainly more vocal, in a good way in my opinion.
Be careful if you decide to have the resonator delete, I went with straight pipes and had an awfully annoying drone around 1800/2000 rpm. To avoid this go with the xpipe or leave in the resonator that basically has a Hpipe installed in its casing.
Another option is to have the stock rear boxes removed, opened up, have the baffling material removed, welded back up and installed. The 55k I purchased come like this from when I bought it. Previous owner had done this to the rear boxes. It's definitely louder than stock, but maybe not as loud as the C63 set up. The C63 boxes will need some adjustments to fit in the E w211 floor plan. Nothing to drastic that an exhaust specialist can't achieve.
Good luck finding the exhaust sound you require. The M113k when has exhaust modifications sounds amazing. In my opinion needs it to some extent. Amazing Engine/sounding car.