http://www.alabbasi.net/450SEL.htm
That is the member who had severe rust repair done on his car. Yours wouldn't need nearly this amount but he has pictures of the rockers and jack points being replaced. By looking at them you should be able to get a better idea of what is involved in the process.
http://www.kkmfg.com/
This is the place that sells aftermarket body panels. They do not have 126 listed in their catalog but they also do not list 123 parts and another member here called them and they just did not have them listed. So, moral of the story is, call and see if they have rockers for a SWB 126. You may be pleasantly surprised.
The jack point boxes (or even the rockers for that matter) can be bought at any MB dealer. They should not be too expensive since there is not much to them.
Thanks
David