Looks like the Briar pants are nylon? so you would need plenty of warmth under 'em. A good winter weight wool is great stuff, as has been said often. Personally, if I'm wearing wool as my "base" layer, I like to wear a very thin silk or breathable synthetic under that. Maximizes warmth and comfort range for me.
A note about the synthetic stuff: poly pro is by no means the only material available, and I gave up on that long ago too. I regularly use a heavyweight polar tech fabric as my warmest base layer ( polar tec "power stretch" ). Those, under a good pair of wool pants, kept me comfortable up here the other day with temps in the single digits, and a breeze added to it.