I'm not certain if this will help but; You can buy quarter panel patches from several companies. One of them is K&K Manufacturing.
Here is a link to the catalog pages with diagram and prices.
Sheet metal parts for 450SL
If your problem is rust it's going to require a lot of work to eliminate it all. The inner wells may need reapair which probably requires fabicating your own patches out of sheetmetal.
Good luck