I run an SLK350 in SCCA,
MBCA and other programs. My competition tires are Kumho V710 245/45/17 in front and 275/40/17 rear. With stock offset front wheels you'll need a 5 mm spacer to clear the suspension. You're running a 55 with 18 inch wheels so the widths should work, but with a lower profile. Keep in mind some manufacturer's 245s are wider than others.
With this tire combination the SLK350 seems to be the most nimble 'stock' Mercedes made.
Ted Joseph
82 300TD
86 190E 2.3 16
05 SLK350