All SLKs will squeak to some extent due to the flexing of the roof as it's not an integral part of the body. In fact all hardtop convertibles suffer the same symptoms.
One known solution is to drop the roof half way and liberally apply something called Gummi Pflege (google is your friend) to the rubbers seals. I've got mine to a point where the flexing is only noticeable at low town speeds over lumps and changes of camber.
My opinion is that the more they squeak the less the previous owner has taken care of it.