Hi, I would stick with the 18", my personal feeling is that 17" are just too small. I have just bought some Brabus VI reps and I am very happy with them. I got 18" but would have liked 19" however with a family the ride of the CLK was important to me to so I needed to compromise. I have the exact same tyre sizes as you 8.5J front and 9.5J rear staggered and I will be changing the pads to get the wheel arches flush and possibly the springs too. So at present the photo shows the standard suspension height.
(Also $1800 for 17" is far too much, I would expect them gold plated for that?)