Hello and Welcome!
My Box is a daily driver. Last winter I purchased some Blizzaks and got along fine. However, when it's snowing like crazy, I stay home. No matter how good the tires are, deep snow will defeat a low sports car every time. And I might be able to get around but the idiot flying along in his 4wd can't stop and I don't want to meet him by accident.
Size matters when it comes to wheel diameter. 17" snow tires perform better than 18's. The wider the tire, the less effective it will be as a snow tire. Also tire companies do a CYA and probably sway more towards performance than snow traction when building 18's - at least a little more than 17's. Since you haven't purchased a Boxster yet, you may want to consider tire size in your purchase. Normally buyers prefer 18's over 17's, so if you find one with 17's it might work in your favor to buy it. But you can get along fine with 18's with a little common sense and a light foot.
You will fully enjoy purchasing a Boxster.
Terry