To answer the questions above:
Yes, the square setup is fine. I often recommend it for those who need the best snow/ice traction. 235/40R18 or 245/40R18 are best for maximum winter traction. 255/40R18 can work all around too. To buy a square setup, just buy four of the "front" size and none of the rear. You sacrifice aggressive looks and the ability to put the power down more aggressively on dry pavement for better snow traction and more simplicity (no need to limit to matching front/rear pairs).
245/40R18 fits a 9.5" wheel. That is the widest it is recommended for, so it will look a bit stretched, but not horribly so. The recommended range is 8"-9.5". So "ideal" would be 8.5", with 9" a close second. (8.5" is the width they put it on to measure all the specs like width and total diameter etc).
235/40's actually have the same width range, so the above applies to it as well, although I would not want to put 235/40 on 9.5" width if I could help it.
I have accepted a new position at Tire Rack and am no longer in the sales department. firstname.lastname@example.org
is available to assist with M3 questions.