The very high performance tires used on the SLK don't last long. You have what is called a staggered tire setup. The rear tires are larger that the fronts. In this case they should not be rotated. You have 7" wide wheels in the front and 8" wide wheels in the rear, so you are stuck with buying different sized tires, unless you change wheels.
It sounds like you need some good all season tires. Tire Rack has a feature called Upgrade Garage which will help you select the tires you need. You can also compare them to each other and read reviews and sometimes tests of these tires.
Just go to
www.tirerack.com and enter your year and make under shop for tires and go from there. You don't have to buy from them, but it will give you a good idea of what you need and some ideas of what is available.