UnderArmor Cold Gear will work just fine as a base layer. The only problem with base layers is that they are more difficult to remove in marginal weather (cold morning that warms up as the sun comes out). I have some "windproof" XC running pants that have a wind and water resistant front and are close fit but not skin tight so I can wear a layer underneath in really cold weather. In marginal weather I wear them over my usual riding shorts and remove them if needed. I only use a base layer when I know I'm going to need it for the entire ride. Knee warmers are a good option if you want something you can take off and stuff in a jersey pocket.
If you don't mind being a bit Fred, wool hiking socks keep your feet warm and it is amazing how much less your legs bother you if your feet are warm.
Last edited by Myosmith; 09-09-13 at 07:27 AM.