I agree, fit is most important, and you *must* try your goggles with your helmet. When I got my first helmet, my goggles were shaped totally wrong for a helmet and I had to get new ones. Nowadays I think most goggles are made with helmets in mind, but you still need to check the fit.
Be aware that even a thin layer (like a balaclava) under your helmet will make a difference - as will growing your hair out / cutting it short. So try out your helmet the way you'd plan to wear it. Helmets are extremely warm, so my guess is that you won't need any extra layers anyway. I was surprised at how much warmer my old helmet was than my hat (after all, helmets are windproof); the new helmet is even warmer.
I agree, vents are important - if possible, get one where the vents adjust without having to take off the helmet. My old helmet has vents that are plugged with rubber strips that you have to remove and stash in a pocket; the new helmet has a plastic switch that opens and closes all the vents in one motion.
Finally, I'm not sure how important / easy this is, but I'll throw it out there. My helmets have soft fabric pads over the ears. I know of two friends who collided, hard. One was wearing a helmet with soft ear pads, and the other thinks that played into the concussion that had her out of work and skiing for several weeks. That being said - I don't particularly care for the styles that have plastic over the ears, nor do I know that having plastic there would have mitigated the injury. Just a thought.