The best advice I can give you is to not spend too much money right away. After you go a couple times, you'll get a better feel for what you want and like about certain items, and what you don't like about them. Then you can start figuring out what new gear to buy. If you're going with more experienced hikers (and I, too, hope you are for a trip of this duration) they probably have old gear lying around that they can loan you until you figure out what works best for you. In fact, it woud probably be a good idea to go on a couple weekend trips before this outing to get a better idea of what you do and don't want to take with you for a longer trip. Like most things we talk about on this board, it really comes down to personal preference.
If you're going to buy a new pack, go to every store you can that has packs in the size you're looking for and try a bunch on. Don't get locked into a specific one too early, because as I'm sure you've already found out there is an endless variety of options. Wear them around the store for awhile with a load similar to what you plan to carry and see how they feel. Once you find one you like, you can look on the internet to see if you can find a better price, but don't buy one online without trying it on.
As a sidenote, I read an article recently that had polled AT through hikers and reported that those who used expensive high-tech tents reported the same satisfaction with them as those who used inexpensive pup tents or tarps.

Good luck,
Chris.