I put some highly regarded Bridgestone Potenza tires on the SLK230K when I first got it (stock 16" rims and size, standard tire sizes). They were absolutely the worst tire I've ever had on any car, wore funny to the point that I felt they were dangerous a scant 2200 miles later. I fitted Continental ExtremeContact DW tires to replace them ... and they were absolutely fantastic. Not rated for snow/winter use, but it's never really cold here, but dang they stuck in the corners, were quiet, and braking was outstanding. When I wore them out completely (about 28,000 miles later), there was a shortage on the rear tire sizing and I replaced them with Goodyear All Season Sport GTs. These worked very nearly as well as the Continentals, have a much longer life span rating, and similar noise/braking/cornering feel.
Then the SLK230K was destroyed. :-(
When I first acquired the SLK280, it had stock 16" rims and a set of WheelWorks house brand tires in the standard sizing on it. They didn't work badly but they didn't work particularly well either. I decided to change them and upgrade the car to the optional 17" wheels from the Sport Kit for that year, and fitted a set of the Continental ExtremeContact DW tires to it. They work great, look better, and provide the same plush but "in contact" feel that they did on the SLK230K.
I would look to which model of the type 170 cars had 18" wheels fitted and in what sizing, then pick tire sizes to match those specs from whatever the factory recommendation was.
The SLK cars are/were always available in a variety of models with different engine sizes and power outputs. The smaller-engined, less powerful models like your SLK200 were often fitted with the same size wheel all the way around, and typically the tires might differ by just a small amount. Moving up the model line, the rims are larger diameter and the widths are wider at the rear compared to the front ... ostensibly to accommodate the additional power output of the engine and provide whatever Mercedes specified as the handling they wanted. Some of the sizings/etc are styling driven, of course, but Mercedes has a list of acceptable wheel and tire sizings that work with the cars' traction control, ABS, and other safety systems. It is safe to take the wheel sizes and matching tire sizes from the higher performance variants on the same chassis and fit them as an upgrade to the smaller-engined models, although most of the benefit is styling.
For instance, on my 2006 SLK 280:
7J x 16 H2
1.34 in (34mm) offset
205/55 R16 91W tires
8J x 16 H2
1.18 in (30mm) offset
225/50 R16 92W tires
Front (optional sport kit rims):
8J x 17 H2
1.42 in (36mm) offset
225/45 R17 91W
Rear (optional SLK 350 rims):
8.5J x 17 H2
1.18 in (30mm) offset
245/40 R17 91W