A Mercedes dealer might be able to offer some advice on specific things to
check on an SLK, but here's my experience with a '99 SLK, sport model:

A couple of years ago, the top was almost up when it stopped, and I had to
lower it again before it would go up completely. I took it in the next day,
and they found nothing wrong.

It had happened while I was raising the top in the garage of the apartment
building where I live, and at the same time talking to the attendant on duty
at that time, who had never seen the top move before. I may have been
distracted enough that my thumb eased up for a second, causing the top to
stop moving. It hasn't happened since (I focus on keeping my thumb pressed
against that button).

Because I live in Chicago, where they plow the streets when it snows, I
don't have much experience driving on snow, but I can tell you that the
winter drive setting, the traction control, and the low gears helped get me
out of a spot when I was stuck in some snow in an unplowed alley. If you
normally put snow tires on your car during the winter, I suppose you'd have
to buy tires in two different sizes, because on the SLK the rear tires are
wider than the front tires.

Driving in the rain? No problem. (Neither is driving in the winter; the
cabin warms up quickly, and I have heated seats besides.)