Have you tried to get an estimate from an MB approved body shop? I had some work done on my last car, sounds like the same amount of rust but in four different places. They got rid of the rust plus painted with paint from the factory for a little under $200. Not only could you not tell there had been rust there, but you couldn't even tell where they has repainted, it all looked seemless.

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired