RJIF
The next 'width' up from 205 is 215.
It depends on the size of the AMG rims you have as to whether or not you will be able to fit wider tyres. I have 15" 7J (type II) AMG rims and at the last tyre change, I was told I could go up to 215 max. (I opted to stay with 205 because only 2 tyres needed replacing, and I wanted to maintain uniformity). Your best bet is to phone/call in to see a specialist tyre dealer who should be able to assess what you have. Personally, I have always found my local ATS-Euromaster to be pretty good with advice, but as with anything else, these things are generally down to site personnel. If you have 6.5J wheels, I would think that 205 is about the widest pattern you could fit on (safely!).
Also, it's not just getting the tyres on the rims, but whether any fouling is likely to occur. I think you should be okay there, because I am just upgrading my wheels to 16" with fitted 225/50 tyres and have been advised by an MB specialist they will be okay. However, I have a facelift model which may or may not have a different wheel arch design to your pre-facelift model.
Good luck!
*Edit - SG CL500 I posted same time as you, but clearly I was not so fast on the draw! What an amazing link that is - didn't know that site existed but 'bible' it certainly appears to be!