You may be lucky. Most body guys will not do rust work anymore. Also there appears to be pretty solid metal nearby. At least in comparison to what we see up here in the real rust belt area. Where there is just iron oxide held in place by the undercoating all too often. This is where you can push a screwdriver right through with little effort.

Also unlike working on exterior areas of the car. You do not need to seek perfection.

As mentioned above, the safety issue here makes bodge rust repair solutions unacceptable in this region. New steel must be welded in.

For example your seatbelt outer bolt is threaded into what mercedes designed as basically a solid rocker tube. When it rusts in this area, and the undercoating means its way worse than it appears as the metal rusts inside the undercoating sandwich, your seatbelt outer bolt is now attached to a floppy piece of 18 gauge sheet metal instead.

The damaged inner connection on the seat as well means in an accident the seat may pitch forward, and the rocker may curl up and out. you could be safely belted in and the whole assembly including you ends up in the dashboard.

I have always felt the undercoating on these cars is both a blessing and a curse. Its indestructible, but once anything gets past it, the rot spreads far and wide without people catching on until its beyond repair

This kind of rust has retired thousands of these cars, its a real pain in the rear to repair properly, and exceedingly expensive to pay to have done.