I owned a 2005 350SLK auto about 3 years ago and I am also sensitive to a hard ride.
The SLK was on 17 inch alloys and with the correct tyre pressures was comfortable, quiet and handled fine - a good daily driver, pretty economical as well. The engine is fantastic however I didn't feel I could really use the engine due to the autobox. The manual on the SLK got a lot of press criticism (rubbery apparently) but I would suggest you try one for yourself, it's very subjective.
When I bought my Z4 (7 weeks ago) I was also considering an SLK again but I found a nice Z4 first and went for it.
My Z4 has SE suspension and I've replaced the 18" alloys with 17's with brand new (very quiet) Dunlop normal non run flat tyres. For me the ride is just right for this type of car (although the SLK was still a little softer than my configuration)
I wouldn't fancy a Zed Sport on 18 inch alloys with run flats but I am probably in the minority !! Please note the move to non-run flat tyres makes a significant difference.