If you're up for buying cycling clothes, here is a chart:
http://www.civiacycles.com/civiaweather.php
Note that even for 20º, their advice is for three thin layers on the chest, none insulating, and one insulated layer on the legs. You stay a lot warmer biking than walking. The most important places to keep toasty are those not generating heat from the biking— hands, feet, and face per taste.

If you're interested in doing things cheaply, for wintry weather over a distance like that, something like a polyester or wool t-shirt and a windbreaker should be enough. Wool shirts are pricy; most big stores— e.g. Target— have cheap polyester shirts. If that outfit is a little chilly even once you get warmed up, look for a wool cardigan to wear under the windbreaker. Things that have full-length zippers or buttons along the front are nice for easy layering and delayering, and also allow you to easily ventilate. A windbreaker and cardigan are better than a jacket because you can strip out the insulation if you're overwarm.