Before you put a lot of ca$h into carbon body part$ for the 300E, consider getting the lightest, strongest wheels you can find (Luke at Tire Rack can help). Each pound of rotational, unsprung weight (wheels) that you can save will be will worth about 10 pounds of sprung, static (body panel) weight. Your handling AND performance will improve, and you will usually have better brake cooling as well.
This means resisting the "22" wagon wheel" trend and getting no larger diameter than 16", no wider than 8". Your 300e won't do justice to larger diameter or wider widths anyway - strictly show IMHO. BBS is one manufacturer of light wheels to consider, but I am sure that Luke has more in mind.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E