Hey Gubbys, best of luck with the conductor plate! You mention FCP Euro, they are probably the most reliable Euro shop on the web and imho,. Whatever transmission fluid they recommend, I would get. I see on their website for the 2004 SLK 320 they are reccomending Pentosin and Liqui-Moly, both should work fine. They offer heavy gaurantees with their stuff.
And hey, worst case you need a new transmission, you can pick up a 722.6 for pretty cheap from a junk yard for about 500 bucks. It is a pretty widely used transmission. Best of luck!