Can't think of a thing to add to the advice already given, except for more explanation of the EN13537 sleeping bag ratings.

I "sleep cold
, so I always look at the "comfort" rating.

Where do you expect to backpack? Look at the low temps for the areas you plan to go--not just the "average" low but at the record lows for the months you expect to be out.. I generally plan for about 5 degrees above the record.

If you plan to be out both summer and winter, especially in areas with hot summers and cold winters, you may need two bags. Of course you can always unzip or even lie on top of a warm bag, but carrying the weight of a zero degree bag for hot summer nights is something else again.

BTW, I'd keep that pack clean and keep the tags and receipts until you have most of your other gear. That seems an awfully large pack to me. With REI, you have a year to return it, no questions asked, if it turns out not to fit your gear or to be comfortable for you when fully loaded. Do wait, though, because if you end up with a bulky synthetic bag and/or a bear canister, you might need that size.

There's lots of advice on gear selection and other advice, including gear lists, on the home page of this site.