Are you keeping it or are you selling it? If keeping it I would put a Mercedes exhaust back on as I'm guessing you have a W210 so ~ 10yrs the exhaust has lasted. You could just buy the parts from Mercedes then get them fitted elsewhere.
id probably opt for merc. the one you are replacing is probably original or changed once at best! unless you plan keeping the car an awful long time its not worth stainless unless you want to change the sound! indie will be pattern exhaust (probably unless they use gen part) and pattern parts never fit quite as well and only last till a month out of the 2yr warranty as a rule (in my area anyhow!) so really depends on MB fitting charge and whether indie is genuine or pattern part!
For what it's worth. My original back box lasted 6 years. MB Replacement lasted 2 - I'm convinced even MB replacements are not as good as the originals.
At that point I had the whole lot replaced in st/st 7 years ago and no problems since. Just dug out the invoice - £400 for the whole system, which was the same as a MB mild steel system. I had mine done by Longlife Exhausts in Berkeley Glos. (no relation and no commissions paid - honest)
I am sure you are correct 300T--the old original boxes were from a different era of steel manufacturing ---MB still use Ernst and the like but they dont seem as strong or well made nowadays. ( a bit like the cars themselves really I suppose )
The bracket can be temp secured with a couple of large jubilee clips --OK until it gets worse or fails-MOT ?--Will give you time to shop around get a new or as new one--lots of these in breakers or accident damaged-with good boxes on for well under 3 figures --?