Boxster S fitment and winter tires

[clueless1]

I've been searching and googling all the old fitment threads and I am still at a bit of a loss as to tire fitment.

I currently have OEM 18" wheels on the car. The rims are pretty shot and I was planning on replacing them. Because I am way up north I will need winter snow tires. I know that 17" are supposed to be better as winter tires, but in order to save some dough I was thinking I would use the 18" rims I have currently and buy 19" rims for the summer. I haven't been able to figure out how dramatic a difference I will note in a boxster using 18" good winter tires (vs. 17")????. I will be doing short haul city driving primarily.

I have having a difficult time finding tires I want in the OEM reccommended sizes (225/40/r18 and 265/35/r18). I was looking at nokian hakkapelita 7s (studded), but they only appear to be available in 225/40 & 255/35. Alternatively, I know someone up here who has LM25s that are a few years old (but barely used, <1k on them) that are 235/40 & 265/35.

Will there be any issues with fit if I went with either of these alternatives? What about tramlining? Anything else I need to know if deviating from mfr recc'd size?