Don't sweat it...

You will likely be on one ticket and one PNR. Depending on where in QC you end up, you may be on the same carrier the whole way.

As for customs, you will clear once in Canada. Going home, depending on your Canadian gateway, you will clear US customs at the last major Canadian departure gateway (YUL, YOW, YYZ) of your itinerary.

As for what to bring, it totally depends on where you go and when. If you are headed to Montreal, it is a major metropolitan city and can offer you anything to buy you may forget. If you end in Quebec City, it too will offer much of what you could need should you find yourself short on something.

I would suggest using Visa for and not bother changing any cash in the US - you can always pull cash from an ATM if you need it.

Enjoy your time. Be sure to have a valid passport for the border.