It doesn't get very far below freezing where I'm at, for the most part it is just wet. Here are the things that I have found to really help in the winter:
Balaclava - mine is fleece, but I've heard great things about merino. One of my favorite things about wearing it is how much it cuts down on wind noise.
Bar mitts - these keep hands warm and dry with no gloves or gloves thin enough to still allow dexterity.
Neoprene shoe covers - my commute is long enough, with few enough stops that I like being clipped in. Neoprene covers over my bike shoes keep my feet warm and almost dry and they have a hole in the bottom so I can still clip in.
A rain jacket and pants - these are more for the wet than the cold, but they do a great job of blocking the wind too, which helps keep me warm. Underneath the jacket I just focus on things that aren't cotton, dryfit, fleece, etc. layer as needed.
One thing I've found really helpful is keeping a log of what I wore, what the conditions were, and how comfortable I was. I don't need to do it every day, but I try to get enough entries so I know what works for every condition I might face. Then when I check the weather the night before, I know exactly what to set out to be comfortable for the next days conditions, even if it's been a while since I last experienced them.
I've only commuted through one winter so far and I tried to make due with what I had without spending too much. I do hope to add some nicer pieces to my kit as time goes on and my bike riding savings stack up.