Originally Posted by Graham

Considering that sheet metal that needs adding here would not be accessible from one side after welding, wouldn't the bare metal be likely to rust again from the inside?

The main structural pieces are still good. But the sheetmetal no doubt adds to the structural strength of the heavier pieces. Weld repair would be my first choice, but not if metal can't be protected after welding.

One possibility would be to weld repair and leave some access holes to spray in rust treatment. That is how rest of car is protected once/year but I guess they (Rust-Check or Krown) don't get at everything, especially where MB undercoating has failed.