I would highly recommend you read
the unabridged guide before your move. I know it's mostly US-based, but there's plenty of relevant information for Canada as well, including cheap VoIP calling options, mobile usage strategies, software, equipment, and other fiddly bits.
As for the major Canadian MVNOs that're going to have reasonable bulk data prices with service in Montreal, your four core providers are going to be
Fido,
Koodo,
Solo and
Virgin. Mobile data isn't going to be cheap, which is saying something given how expensive it is down here.
Learn how to go on a data diet, and take advantage of WiFi hotspots and your campus WiFi network.
For a reasonably full list of known Canadian MVNOs, I've got them
here and you can always check
HoFo (look under both
Canadian Carrier and
Canadian MVNO). There's cheaper than those four in other areas, with the cheapest mobile provider with US/Canada calling and 5GB of data is Wind at $50CAD/month (an extra $10/month gets you US roaming as well), but unless you decide to relocate to Ottawa or one of their other supported cities, it's simply not an option. I only mention it as an academic point.
As to your other question, as someone who's gone through a few winters up in that particular part of the world (including one heinous January in Montreal where I walked eight blocks in a snowstorm from my hotel with pneumonia and strep throat to see a doctor at a clinic, and walked back), I encourage you to
learn to dress in layers and keep dry. Invest in good thermal underwear (wool or something breathable), find a good parka, get a toque, a decent pair of winter boots and some winter socks (no cotton), and a good set of fingerless+mitten gloves. The rest is downhill and can mostly be regular clothing (some of that should be core wardrobe anyway), and after enough time out in the cold, the frost kinda kills the nerves in your skin. As for cheap places, there's your currently local Goodwill or Salvation Army, and there's plenty of thrift stores up there as well, which includes the
Salvation Army. Be frugal, but don't be cheap.