Your Apex wheels should hold up with salt and snow without any issues. Clean them once in a while. The big issue will be the summer tires. On snow or black ice, it will slide around. Cold winters will also be outside of the limitation of summer tires and the rubber can crack.
Depends on your oem wheels, but winter set up are most of the time square. And they are cheaper in 17" than 18". Even 17" winter will clear a 335i brake calipers. Since winter aren't used that much, I would do a square set up. I used to switch front/rear every season.
Our winters are weird, one year it will snow like crazy every week, another it will snow one day out of the entire season. If you drive alot especially to work or so, get winter tires. What I used to do was when there was a few inches if snow, I would go to an open parking lot, and test my limitations. Turn off dtc helps to give more torque to the rear. Drive slow and avoid the crazy va/md drivers that seems to think this is the first time they have seen snow.
I used to have the rial salerno 17" with blizzaks, because the wheels are cheap and easy to replace, and the 5 spoke made cleaning ice and such really easy.