I've been looking into a '05+ W203 sedan as a far-on-the-horizon replacement for my 300E. Mbworld's W203 forum is a decent place to get info on the chassis, but there's a lot of young tuner-type posts to sort through. Some of what I've found:

-The '05 C230 gets better gas mileage and has more low-end torque, if in-town driving is important. The V6 in '06+ cars is much more refined, trades less low-end torque for power higher in the rpm band, and gets worse gas mileage. And it has a better exhaust note.

-The only complaint I ran across with the 7-speed is that it's not very well suited for the low-torque 2.5L V6. Lots of complaints about gear hunting, and just feeling sluggish compared to the 5-speed/1.8L I4. Doesn't seem to be as big of a problem with the 3.5L.

-The C350 should be very fast, and may even get better highway mileage than the C230 because of the taller gearing.

Anthony