You could just get the OEM battery from your local Mercedes-Benz dealer. It'll probably run you $125 and you're guaranteed to get the right output / size. I just had to replace the Batt in my C32 and it was only $125. That's less than almost any store carrying a compatible aftermarket. That's most certainly what I will do when mine gets flaky. The electrical systems in Mercedes-Benzes are so finecky it makes a certain amount of sense just to get the original design-spec battery. Not saying that you can't find a comparable one, but it'll save you a lot of effort.

Also, on that same note, if you need OEM parts for the crossfire and need to pay for it out of pocket, you may as well get those from Mercedes too, they are much cheaper than from Mopar. Nearly every part on our cars has the Mercedes part number stamped on it, just call up the dealer and ask for it. Almost everything will cross-ref to a 2004 SLK. Part numbers will start with "A 170 ..."