Your conditions may vary somewhat from what I am used to, as I am in a very different part of the country - California - but I have spent a lot of time out in all weathers in the Sierra, and the temperatures you mention are pretty familiar to me. One thing I will start with is that I find it easier to get too warm than you might expect if you are not used to those conditions, and you have to pay close attention and adjust your layers as needed BEFORE you get too hot or too cold.
So here is my arrangement:
I start with very light wicking base layers. I have used both wool and polyester with success - currently it's mostly polyester as I've had durability problems with wool(plus wool costs more). When you look at catalogs or websites you'll usually see various weights of base layers - what I use is either called "lightweight" or "Silkweight" (but note, is is NOT silk). I wouldn't use anything called medium weight for any active pursuit such as hiking, XC skiing, or snowshoeing.
Next step for me is a light shell layer. For pants, I currently use home made pants (EPIC fabric), but I have used Gore-tex pants in the past with success. I have side vents on my pants always - full side zips would do the same thing but are heavier. I consider the vents essential to prevent overheating on the uphills.
I never wear more than just those two layers on my legs while on the go, down into the teens which is as cold as I have to deal with. I often go without the shell pants on warmer days if it is sunny.
On top, I use a Marmot Precip jacket. It could breathe better, but it vents well, keeps me dry in wet snow, is pretty light and pretty cheap on sale.
I also have an expedition weight long underwear zip-t that I use as a mid-layer. I sometimes use it over my base layer, and sometimes I use the shell over the base layer. Often I am in just the base layer shirt. On the coldest of days, I might use the shell and mid layer together while moving, but that is unusual.
For camp, I have polarguard insulated pants and anorak (homemade). Your light down jacket is probably close to my polarguard anorak, and light down pants like the Montbell or Western Mountaineering pants would be close in warmth to my pants. Thick fleece pants would probably be warm enough, though not quite as warm as the insulated pants, plus heavier and lots bulkier, but they can be had cheap.
Hands and head: CRITICAL. I use powerstretch glove liners, homemade knit mittens, and Paclite mitten shells, combined in various ways. I take two pair of the liner gloves, having lost one glove once in the middle of a week-long trip.
On the roof, I use a lightweight balaclava (like lightweight underwear fabric) and a really warm fleece hat. Plus a sun hat for the nice days.
This works for me in the temperatures you mentioned,but your needs may vary. I seem to generate a lot of heat while on the move, and need less clothing than some, while when sitting around I seem to need more than some do. Go figure.
In general, I would say stick to synthetics for stuff you'll wear while active(down is fine for in camp or at lunch), and if in doubt bring more thin layers rather than one thicker one, and at first it's better to have too many clothes rather than not enough until you know what really works for YOU.