Op I'll be there too in Jan! It's a great time of year, often very crisp and bright as well as cold. The forecast for the month doesn't look too bad - cold at night, just above freezing in the daytime.
Silk thermals all the way! I rarely feel too warm in them, there's no bulk, they don't get clammy, they take up no room in luggage etc. I've been known to wear them under tights. Merino thermals are warmer if you plan to walk around all day but it will be overheated indoors so merino may just create another problem.
I have a down gilet from Uniqlo which fits under even my most fitted coats, can be packed into a very small pouch when I'm not using it, weighs nothing and is great for keeping the wind out. If you have a coat that's big enough for a chunky jumper I'd suggest buying one of their thin down jackets to wear underneath, it will solve a lot of problems and they're not expensive.
I like earmuffs because they keep the wind out of your ears without giving you hat head, but in Jan you'll probably need a hat as well. I find if my torso, hands, head and feet are warm enough and I'm dry and moving about I can tolerate it. And there's always somewhere to get out of the cold for a bit when you need it.
Have a great time!