Hi.
I purchased my 77 350slc from near SAS
in NC..
It had very little rust.. only in parts where water was accumulating.
I guess the fact that MBs don't rust as much is a combination of the paint primers used, the thickness of the sheet metal,
and other factors..

The metal used in the MBs (at least the older ones), seems to be thicker than the paper thin sheet metal used in newer cars..

There is nothing like the vault-like sound when i close the doors in my car!