It’s rare to need more than a couple of layers up top. There’s so much high-quality cold-weather gear on the market these days that it’s hard to go wrong as long as you go with the goods from a well-known company. I’ve become a big fan of merino wool gear, which insulates fantastically at a wide range of temperatures and doesn’t stink even after several runs.
Down below, the biggest concern is keeping the family area warm. I’ve done a polar bear dip in which I submerged myself in the ocean in Maine on a cold, windy New Year’s Day. The discomfort of that was nothing compared to what I’ve felt the last few miles of and the first half hour after longish runs on brutally cold days. On really cold runs (under 15 degrees) or it’s especially windy, I wear wool briefs. Nylon ones with a protective panel in the crotch also work well.