Go to ASP Racing
http://www.aspracing.com/frame.htm
.. and then under 'street pulleys' select 'Imports'. Scroll down to Mercedes and you'll see the kit for a '98 - '00 C230 (includes pulley, MAS relocation pipe, plugs, fuel pressure regulator and belt). It should fit a 98 SLK. I had a similar kit on my '99 (from Linh) and it worked great. A little pricey @ $1,200! Kit #ASP331320