Our first baby is due in November , as is my C200 AMG Line , and with it being a relatively new model i'm struggling to find any information on which ISOFIX baby seat bases will fit it.
Although it's supposed to be a universal system, apparently differing seat shapes mean that not all bases fit all models of car.
Retailers recommend bringing your car along to test fit, but as I don't have the car yet i'm not able to do that.

W205/S205 owners with baby/toddlers, can you offer any advice?

Cheers.

 

See the ones offered by MB (they have a full catalog), then if you want to save a bit, there are similar ones by various vendors (at different prices).
ISOFIX is indeed standard, so anything with that label should fit very well (just a matter of your personal preference)