As far as I know, the car should take platinum spark plugs. My C280's take the platinum plugs, but I do believe that you can settle for something less expensive like copper ones. The non-platinum plugs would have a trade off of being cheaper with less durability and perhaps less performance. I have some links for help with this:
Rock Auto- Plugs for 1994 C180
http://www.rockauto.com/catalog/x,ca...,parttype,7212
Photos of spark plug chaning:
http://www.benzworld.org/forums/w202...ug-change.html
Video of CLK230 spark plugs being changed (it's not the same car, but still an inline 4):
https://www.youtube.com/watch?v=P8SftiS05VI