One other standard bit of advice I've not see yet (in the good advice given so far) has to do with all of the gear you'll carry in the pack.

If you think there's much chance of changing that gear mix significantly, and in particular if you think you might over time want to cut back on weight (and bulk) of gear either by replacing with lighter versions and/or simply doing without some stuff --- then you might want to borrow or rent a pack in the interim. It's better to have your gear list trimmed down to what you'll actually carry on your trips before selecting the pack. Especially if the weight of the pack is a significant factor for you. Note that pack weights vary incredibly, some weigh about half of the total of everything I typically carry (to include my pack).

If overall pack weight isn't much of a concern for you then "never mind" --- just get a comfortable pack with enough capacity for any trip you can envision.

In terms of saving money: consider buying used (this and other backpacking sites have buy/sell threads for that) and watch for particular sales. Ideal would be to identify the particular pack you want and then just watch for some time in various places for a good price. Apart from major places like REI, there are a number of such, including sierra trading post, spadout.com, and the cottage industry manufacturers (ULA, Gossamer Gear etc) sometimes have sales as well.