Get all your other gear together before you start shopping for a pack. You want to buy the pack to fit your gear, not the other way around.
If you're on a budget, there are a number of threads here on low-budget alternatives for light-weight gear. There is a recent one in the Beginners section. When you search, be sure to change the parameters from the 1-week default to 1 or 2 years.
If you're not on a budget, for a sleeping bag go for Western Mountaineering, Feathered Friends, Nunatak--a bag from any of these, properly cared for, will last a lifetime. If those are too pricey, get a cheaper models (like the Campmor 20* down bag) and start saving your pennies. The lower-quality down bag won't last as long.
A couple of suggestions: First, read the articles on the home page of this website. Lots of good info there on gear selection. Second, go
here.. There are lots of links to low-budget (and not-so-low-budget) lightweight gear recommendations, gear lists, etc.
Remember that price is not the sole consideration. Shoddy gear that falls apart in its first or second season (or worse yet, in the middle of a severe storm) or heavyweight gear that you want to replace with something lighter after your first trip or two is not going to save you any money! Consider looking for used gear. There are always people who want the latest and lightest innovation and are selling their older (but still lightweight and high quality) gear.