Tires for Comfort
I think the 19" are the right size for the 221 in terms of comfort and ride. Since a sporty look or getting more grip on the rears isn't a priority I'd go with a square set up vs staggered so the tires can be rotated.
For tires, having tried most of the less expensive options over the years, I always come back to the Michelin Pilot Supersports. Nothing comes close in terms of comfort and smoothness. They have a 35,000 mile warranty when run square (half when staggered). Everything else got chewed up on the rears in under 10k miles, while the fronts were good for 20k max.
Here's a good source of info on verified tire size fitments on Mercedes:
http://www.alloywheelsdirect.net/inf.../mercedes-benz
Looks like 19" x 8.5" wheels and 255/40/19 would be the OEM fitment for 19's and 18"x 8.5" would be 255/45R18. On the chart, OEM fitments show as "Mercedes" or "AMG".










