Merino wool baselayers up high and down low.
Then a few layers of regular wool up high with some good insulated pants down low.
Top it all off with a set of oversized Frog Toggs if it's going to rain.
Pay the extra dinero for a good pair of boots (buy them a size too large to allow for double wool socks) and some expedition quality mittens and you'll never be cold and wet again.