I was faced with a very similar situation, but it was driver's side rear pan , in a 123.
My mechanic sub contracted the job out to a body shop he trusts. They did an okay job, albiet a bit rushed looking at close inspection underneath. The job cost about $250. But after looking at it, you can definatly do it yourself, if inclined. I am sure cost also depends of the severity of the rust. Mine was a corroded (sp?) area about 4" around, nice view of the street when the carpet was lifted.
File, sand or cut out the really rusted area.
get some screening or solid sheet from an auto-supply.
under body putty and the like, I have a product called POR-15. they also sell a rust inhibitor and chassis-black product. It is fairly easy stuff to do. I am skipping alot, but do a search and I am sure you will get more complete advice than I am giving here. But if you have the time, its kinda easy.
Hope this helps.
85 300SD 285k
Charcoal Gray/Grey MB-Tex
79 300CD 142000mi "Rabenshwarz"
Black / Black MB-Tex, Burlwood Int. TOTALLED - 10/24/02 --