The short answer is, if you need to ask all these questions it's time to hit a library.
I'll speak from my experience, though, which goes to -12.
First off, down is great, but bag condensation is a thing. Depending on how long you're planning on being outside or whether or not you have access to a cabin, you might need treated down or synthetic. Down has the highest warmth-to-weight ratio, but Synthetic is not too far behind. It's slightly heavier, but can be just as warm (in my experience). If you can dry out your down bag in a heated tent or cabin without it freezing, it's gonna be the better choice, but if not, you'll want some resistance to moisture. Inside a snow cave or tent, moisture builds up pretty quick.
Now, as for the legs, in temperatures like that you NEED insulated pants. Don't mess around- get insulated everything. Your gloves should have a synthetic or down fill. YOur pants should be like a puffy jacket. Your boots need to be insulated boots. Your socks should probably be pretty thick, too.
Take the temperatures seriously. This isn't the place for baselayers and wind pants, unless you'll be in a cabin all night.
You'll also need a full face mask, goggles, and a warm hat. I can go on and on with a laundry list of things. Insulated water bottle holders? A pot large enough to melt snow?
Best give us some more details, or ask more detailed questions.
Hope this helps!