Welcome to MBWorld and grats on the w204. For wheels, what widths are you planning on running as that will factor in with fitment. In general, you're going to want to be in the mid 40s for front and upper 40s/low 50s for the rear running staggered. Most go with 19s in that chassis... some have run 20s, but it sits kind of high unless you plan on dropping her.
Let us know if we can help you upgrade your lighting to pure white. Eyebrows, low beams, rear plates, reverse lights, signals, and interior are pretty common updates on that car. We have an in house C300 also so very familiar with her.