Call the dealer, and ask who they use. That's what I have done.
You do not want some grunt who knows nothing about MB screwing up
your car. After you find the guy, then ask him whether he has the proper
clips for the trim in case one breaks. If no then someone needs to buy the kit from the dealer. I have heard stories of idiots gluing the trim strips and the roof trim.

My experience on 2 different w-124's is that the guy shows up and does the
R & R right in your driveway. You then need to not drive the car for 24 hours to allow the sealant to cure. It's also important that they use the correct sealant to glue the glass down.

In my opinion paying extra for the Sigla glass is silly. He will probably put in
PPG. Installed cost was something like $250.00 bucks. I play golf with a guy who is GMgr of a glass production plant. I asked the question about differing hardness of OEM versus for instance PPG. He smiled and explained that glass is glass.

Steve