You did one thing very right: you went to a shop and tried packs on. Keep doing that - but as others will tell you, do that after you have the rest of your kit assembled. The weight and bulk of your gear, plus several days' worth of food, will determine the size pack you need, and the suspension it should have - which will determine the weight of the pack itself.

Take the rest of your gear with you when you try on packs. That lets you make sure that your gear will fit in the pack, and that the suspension will carry the weight.

When you're looking at packs, I'd recommend an adjustable suspension (i.e., you can shorten or lengthen the spot where the shoulder straps attach to the frame sheet.) this will greatly increase the comfort of the pack. (Don't rule out fixed-length packs - just be aware that they are less likely to fit you perfectly.)

As far as lighter, good brands, ArcTeryx is good; so is Granite Gear (my personal choice), Osprey, Deuter, and several others - in fact, if you stay with the "better" brands (the ones frequently discussed here), it will be hard to find a bad pack, as long as it fits. There are plenty of 3 pound packs out there that will carry 45 pounds comfortably in 50 - 60 liters of room.

No pack is waterproof! The only way to keep your gear dry is to pack your gear properly. Either store things in zip loc bags and use a pack cover, or consider using a pack liner - a waterproof bag with a roll-top closure that fits inside the pack. (Again, my personal choice is silnylon stuff sacks inside the pack with a pack cover.)

As far as the rest of your kit, which you may not yet have, if you want to simplify things, choose MSR and Thermarest. It's a bit pricey, and it's not the very lightest, but they make reliable, almost bombproof, gear that is reasonably light and easy to use. (Send me a PM with your email, and I'll send you two lists - one using their lightest and most expensive gear, and one that's a step back: two pounds heavier and a whole lot less expensive.)

Good luck.