SLK32s have 2 piston calipers. Opposing pistons on the rear, and same side pistons on the front.

I am convinced that PADS are the main thing when it comes to brakes. There are huge differences in initial bite, hot friction, cold friction, dust, noise, and rotor friendliness between different pads. My personal favorite is EBC Redstuff, but people have different tastes. I have bought "performance" brake pads that had less friction than the OEM pads. :/

Braided stainless lines made a huge difference on my Supras, but less on my SLK32. The one time I bought slotted rotors I could tell no difference.