Problem here is that you are not the first owner and the rust may be due to earlier damage that has not been properly repaired.
Helps if you have full MBSH BUT IT IS NOT essential. MB have dealt with rust even when the history is a bit patchy.
BUT if you have a service due soon, I would have that at an MB dealer and then raise the rust issue.
If not, you need to get to an MB dealer and say you want to claim under the 30 year rust guarantee. They will send details to MB or thye will send you to an apprioved body shop who will send details to MB and see if they will pay. Either way, I'd get it fixed at an approved bodyshop so it is properly done and so as to keep the guarantee intact.