Sept. 30-Oct. 5 can certainly be coat, or at least jacket, weather. Also, make sure you have a small umbrella packed with you.
I am not sure what I can say about the delights of flying with Air Canada. Generally speaking, flying in Canada is more expensive than in the U.S. They do occasionally offer seat sales, so by all means check their website and arrange to be notified if the seat prices go down past a certain point. They now charge to check bags in Economy.
In the interest of full disclosure, I have to admit to really disliking the Toronto Airport--my husband refuses to fly through it because he finds it too stressful.
Another option you might look at is flying to YUL (Trudeau Airport in Dorval, not far from Montreal) and then taking the train to Quebec City. There is a free shuttle from YUL to the Via Rail (passenger train) Station in Dorval. You can also book Via Rail train travel online. Montreal is about 2-3 hours by train from Quebec City, and YUL is a larger airport than Quebec City's airport, so you may have a better choice of flights. Plus, taking the train is enjoyable, and you get to see some scenery. If you want to splurge, you can take Business Class (Via 1) on the train.