Hi, It should be Autumn nearing winter or the on set of winter during end Dec and beginning Jan. The weather there should be about 10 degrees or so. Unless you are going to near sea area like dan-shui or their lake area, which could be a couple of degrees lower, the temperature should be quite alright. So, a thick cotton or knit longsleeve tee with Autumn jacket should be ok. Wear another short sleeve tee inside if you are afraid of the wind. And, don't go out during typhoon.
Taiwanese are very stylish. Especially in Ximending, you will see quite a number of gals in boots. BTW, in Taiwan, though the stuff is not cheap, their quality are quite good. They have small individual apparel shops that sell apparels that are pretty stylish. So, be sure to pick out some when you are there.
Talking about what to wear, well, I happen to have a couple of Taiwan jackets for Autumn, that I am helping a retailer clear at a fraction of the original price. Very stylish. This retailer sells Taiwan design apparels and normally clears her stock frequently to keep her customers happy. So, the stocks that she retires are quite current stuff. You may wish to take a look at my blog at june-myfashionbusiness at blogspot. I also have a banner in this Fashion portal called FashionInfoHub dot com. This fashion portal has updates of the most current fashion sales events and wholesalers information. You may wish to sign up for their news alert to get the updates of the fashion sales on going currently.
For further info, PM me.