I'm not sure where you're located, but MEC in Germany makes total replacement units from the cat-back, all stainless with hardware included. I plan to go this route and have a hi-flow cat installed just after each downpipe.

I'm not sure if the US model inline-6 has a single downpipe vs separate ones. This is the setup on the V-8s (US vs Euro). Check and see if MEC has an option for your vehicle. Otherwise a reputable local shop will be able to fabricate (especially if they have a mandrel for non-deforming bends) and add mufflers. A good number of MB owners have put Magnaflow mufflers on their cars.