Contrary to what many think, there are only a few things that you need for winter hiking. There is a big difference between clothes for standing around outside and those for hiking. For standing around you need warm clothes; for hiking you need to stay cool because if you sweat, you will be cold. I found dressing for cross country skiing at 10 °F in Canada is a real challenge; surprisingly you only need a few things.

Let me first say what you don't need.
One, you don't need any heavy clothes. You want to dress in light snug, slightly loose rather than tight, fitting layers. The air between the layers, not just the materials, helps to insulate you. I find heavy sweaters, coats, shirts, etc problematic because they make you sweat. For the most part I wear the same thing in the summer, spring, and fall that I wear on the coldest winter day.

Two, you don't need any special shoes or socks. The stuff you wear in the summer, if it's decent, will work well in the winter. A big mistake is to wear heavy winter socks that make your shoes tight. Tightness prevents circulation guaranteeing that you will be cold.

Three, make sure you do not wear cotton socks of shirts or undergarments. Cotton holds sweat, making you cold even if you ware warm clothing. Wear synthetic socks: I get mine at the dollar store. Wear wicking shirts: I get mine at the Northface $20 store.

A few things that you definitely need.
1. A light weight balaclava, face mask, for real cold weather. Prevents your face and neck from getting cold.
2. A good pair of gloves that allow you to move your hands.
3. A light material hat that can be pulled down over your ears.
4. Gaiters for the snow. These fit over your shoes preventing show from getting into your shoes. They come up the to mid calf or knee.

What you should have already.
1. A light weight shell or jacket.
2. Wicking t-shirts, so if you sweat you won't get cold.
3. A very very light sweater or fleece. Heavy stuff will only make you colder.

I highly recommend that you purchase stuff at REI for several reasons.
One, you will save a lot of money in the long run.
Two, the only sell high quality stuff.
Three, you can take back anything you buy at any time without being hassled. I've taken things back years later when they just did not work right.
I should also add that most manufacturers of high quality equipment guarantee the products for life. I once received a $300 credit for a new coat, 10-15 years after the initial purchase!

On line I highly recommend Campmor. Prices are slightly better, but no other store has the favorable return policy of REI. I always try to purchase items at REI, because I know that it is safe- I can always return it.