I agree with the information Glenn provided. If you expect there is a chance of temperatures being around 0 F I would recommend a bag rated for -20 F. If you are a cold sleeper like me, you can sleep with more of your clothes on. I was recently on a trip where it was 4 F and I was plenty warm in my -20 F bag.

Always where clothes in layers and keep in mind moisture control (sweat is your enemy). If there is a possibility of wet conditions then stick to synthetics and wool. Wool insulates when wet.

Another tip for water is to bury your water bottles upside down in the snow. Snow is a fantastic insulator and will keep your water mostly liquid. Tie a lanyard to the bottles to make finding them easier.