Welcome to my club
I own a very rusty 450SEL that I am in the middle of repairing and my car is pretty much suffering from rust in the same places.
See the attached link for pictures:
http://alabbasi.netfirms.com/www/450SEL_pic.htm
Currently I have purchased the fenders from another lister, the rocker panels from the Mercedes Benz Dealer (at a cost of $600 dollars). And the rear quarters are currently getting cut off another 450SEL at a yard 100 miles away from where i live.
As you own a 280SE, you can still get aftermarket rocker panels at less then $100 dollars each. You can also get wheelarch repair sections.
Once place to find these parts is
http://www.rustrepair.com. Good luck and post some pictures. We would love to see the progress your making.