The must-have for me when it comes to winter riding is a good pair of goggles! And not just ones that cover your eyes. Make sure you get the big ones that you see skiers wearing. Trust me, you will thank me later. It will keep the wind chill out of your eyes and make your ride much more enjoyable.
My wife LOVES the EMOJO Breeze! The battery is right under the seat and that helps keep it balanced. 500w/36V gives it good power, but it won't overwhelm you on your gravel flat trail. Plus she loves the colors.
Another good one she likes is the Phantom Swirl... has some good power, but is also...