Whatever you get should be 100% water-proof because they're non-breathable. The air-movement is what kills you. I use Helly Hansen poly-urethane pants but I'm gonna have to NOT recommend Helly Hansen because I've had to repair the seams multiple times to the point where I don't even care anymore, the top elastic is only holding on by a few threads. But they sure do wonders for keeping my legs warm.

Most of last winter I got away with just those over my work pants - only on the coldest teens/single-digit days would I wear thermals underneath my pants.