If you really expect 0* temperatures and sleep cold (or are a female) then tipi is right, buy a bag 15*-20* warmer then you need. You also need to realize that a warm bag does you little good if you have an insufficient sleeping pad. For those temps you need something that has an R value of at least 5. You can get that from 1 pad like the Thermarest NeoTherm or the Exped Downmat UL7, or you can stack pads like a Thermarest Prolite Plus and a Ridgerest.
Most sleeping bag manufactures rate their bags (even women's bags) for a fit, young male. As you get older you tend to sleep colder, and women tend to sleep a bit colder then men.
You also want to make sure that you get a bag that has the right shoulder girth. The Kodiak, that you mentioned is a HUGE bag and if you are of slight stature, all that space will take forever to heat up.
I am a big fan of Western Mountaineering bags, but Feathered Friends, and the top bags from Marmot and Mountain Hardwear are pretty good too. Mont-Bell bags are interesting as well. I have never used a Mont-Bell bag, but I have used a lot of their down gear with excellent results. If you are looking for a deal, Golite has a -10* rated bag that is only $399.
Then you also need to realize that a sleeping bag this warm will absolutely roast you in summer, so you will probably need at least 1 more bag for year round backpacking.
Backpacking at or near 0*F is no laughing matter, those are serious temperatures that require serious knowledge and specialized (and expensive) gear. Make sure you are safe and have fun.