I would talk to the dealer you might get some good will. They have been fixing rusty W210's.

As for the W124 find a good set of rust free doors. I have seen a few MB's with rust in their it starts under the plastic you are only seeing a small part of it. I bet their are holes in the doors.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200