Here are my thoughts (which I know some people won't agree with). But, we are just back from our Winter Camporee here in WV (not the most extreme in the country). Took 5 Scouts - none had winter camped. Spent part of the meeting each week for over 2 months preparing. Camped near the road. On arrival, between 1 and 2 feet of snow on the ground. Low temperature Friday night we were told was around 12 degrees F. Unfortunately it got up into the high 30's during the day Saturday (yes, I really want it to stay below 25). Low Saturday night was around 22 or so.
So, what do I recommend? Use COLD (Clean, Overheat-Don't, Layers, DRY!). I didn't get cold - many of the Scouts did. The biggest reason - they didn't stay DRY. If you get wet, you will get cold.
Wool socks - absolutely. I used 3 pair.
Insulation from the ground - closed cell foam pad, extra blanket on top of that. Some of our Scouts used cardboard. Newspaper is also a good insulation. Cardboard and newspaper have be have a waterproof ground cloth so they stay dry.
Layers - so you can adjust and not sweat. Outer layer has to keep you dry. I had nylon ski type pants.
Separate sleeping clothes - I used polyester exercise pants and shirt
Warm sleeping bag. I added an extra fleece blanket under me.
Many people eliminate all cotton (it does hold moisture as someone mentioned, but so does wool - what it also does is looses most of its insulation qualities when wet, which wool does not do). I still used some cotton clothing. Just have to keep it dry.
Are you getting the main theme here - DRY.
We don't worry much about what melts around campfires - because we don't have them. Leave No Trace is hard in the snow for one thing. The other thing is that if you are properly dressed you are insulated from the heat of the fire anyway.
I bought nothing special for this Camporee - but I admit that we probably have more cold weather clothing already than you will.
We had a great time, Scouts got first place in the improvised shelter competition. They did get cold, but they will do better next time.
Eagle Scout - 1973
Wood Badge Beaver - 2005