Usually I prefer to go just 1 size down ... meaning in your case, 17" would be ideal.
Used rims & used tires can be found in Kijiji and here
http://www.e90post.com/forums/forumdisplay.php?f=300
Winter tire size (proper fitting) would be 225/50/17 H or V speed rated, if possible as to maintain the stock spec.
Within the winter tire grouping, there are the winter performance tire and there are the snow & ice tire. Both group of winters have their advantage & disadvantage. In general, given the T.O. past couple of winters, I feel you should be alright by choosing the winter performance tire as they handle better in dry cold winter weather.
http://www.apa.ca/WinterTires2011-Performance.asp
Since you are trying to buy used, then if you are able to find a set that are mentioned in that link, along with priced reasonably, then you should consider it.