Good job. When you think about it, Mercedes had the good idea to mount the battery on a removeable plate. In case the plate rusts, you can just replace that plate. I had some other cars where the battery was mounted directly on the frame rail (like VW MkII) and when that rusted you had to cut and weld.

One trick if you ever find rusted parts like this: fill a tub with regular white vinegar and immerse the part in it for a couple days. When you take it out it will be completely stripped and all the layers of rust will flake off if you hose off the part. Make sure to grind/sand and paint the part directly after or coat it with something. I find that putting POR-15 directly over rust almost inevitably makes the rust come back after a while.