Welcome to the Forum.
If you are going to get a set of dedicated Winter tires and changing to 18" wheels, I would suggest going with a 235/65R18 tire. A narrower tire is best for driving in snow and slush since there would be less resistance and hydroplaning. That size is also an exact match for the 20" you have now. I'm running 235/60R18's Blizzak DM-V1 that I used on a previous vehicle but will switch to the 235/65R18 size when I have to replace them. I'm using the 18" Ford steel Interceptor wheels. Not sure about the offset since I didn't have to worry about it using the Ford OEM wheels.
Peter