DC area winter tires or not?

Hello everyone, I have a 2011 Z4 with Bridgestone Potenza 18inch tires on. Bought it earlier this year in the spring. From all my research so far, i have concluded that I probably want to change to winter tires or MW tires for the winter. So I want to ask same the question specifically for the northern VA/DC area. Assume average temp is at or below 45 degrees F for the months of Dec, Jan, Feb, March.

1. Given the weather here, do I need to go to winter tires or would multi-weather make more sense?
2. Do the full winter tires wear very fast if you end up driving on primarily dry roads?
2. Do I really need to go for new wheels as well (if I want to go to 17inch winters) or should I just mount winter 18 inches on my current wheels. In other words, do I really need to go to minus 1 tires and will changing the tires damage my current rims.

Appreciate your help folks...