Backpacking at that time and location is
very advanced winter camping.
Do you have lots of experience tenting out in the cold and snow? If not, I suggest you get that experience before going on the to far more demanding activity of backpacking.
If you have that experience, we should have only a little to add to what you already know.
If the choices of polar and antarctic expeditions and the military are guidance, wool is good and polyester fleece and polyester fill are better - and down is totally out.
Polyester fleece is so abrasion resistant that it is showing up in quantity in resale shops. I got a North Face vest for $5.00 in the Salvation Army in Franklin, PA a week or so ago. It needed a wash but had no wear I could see.
SOFT SHELLS
This term is applied to garments so different that they have only sleeves in common.
A good "soft shell" of Schoeller Dry Skin is the type of outer garment selected by mountaineering professionals. I mention the Schoeller fabric becasue it has tested best and is presently used only in "name brands." By universal repute and from my limited experience, it is highly water and wind repellant, breathes better than any "waterproof/breathable" garment , drys very quickly, and insulates some if used as an inner layer. It is not for steady, heavy rain. I have tried it once under a poncho and so far, so good. It kept the condensation from the inside of my poncho off me. I tried it once alone in a steady, light rain, and it kept me dry for an hour, at which point I came in. Cost? I got mine on sale for $129.00 (Google:thumbup

. That ain't cheap, but you can pay lots more for less functionality.
Other "soft shells" are only lightly water repellant "wind-breakers." Such a garment is useful in dry (or almost dry) conditions where a "waterproof/ breathable" is too hot but you need to keep off the wind.
BOOTS
Boots are a special issue in winter backpacking. Youi need all the function of a summer backpacking boot (comfort, stiff counter, stiff shank, traction) + the correct level of insulation and probably water resistance. I like Mendels 'cause they fit the shape of MY foot. You?
(Do you speak gaiter? If not, go back and read the first two paragraphs of this long-winded post because the answer was "no.")