I also second what was said about "lightweight" packs. Stay away from them!!!!
I would stay away from the ultralight ones without frames and all that unless you have the requisite gear, but you can get a full featured pack that weighs in around 3-4 lbs with a frame, excellent materials, and suspension substantial enough to handle 30-40 lbs. I use an Arc'Teryx Axios 50 and can carry ~30 lbs and still be very comfortable. Osprey and Gregory have similar packs.
Of course, like others said, you have to have the gear to go with the pack. I couldn't get away with that pack if my gear was still the heavy and bulky stuff I had a decade ago. My first real backpacking pack was a Gregory Forester, which weighed about 50 lbs and was crammed to the brim when I had a full load of gear in there. Getting small and light and still staying comfortable (sleeping pad, sleeping bag, hammock/tent, etc.) can get expensive real fast.
Sleeping bags will put you in a similar boat. Assuming the same temperature rating, getting smaller and lighter will be more expensive. You can get a bulky synthetic 32 degree bag for $60 or you can get a ~1 lb Nalgene bottle size Western Mountaineering down bag for $300, with a lot of options in between. Down is generally more expensive than snythetic, and higher fill ratings of down are going to be smaller and lighter, while being more expensive.
Synthetic will still insulate you about 60% of its dry effectiveness when wet, where down loses its insulation properties when wet. In reality, being in a wet sleeping bag sucks hard either way, so keep your bag dry. Keeping it in a dry bag and using a little bit of common sense will go a long way. I have never gotten a down bag wet and I have yet to go on a trip where it didn't at least drizzle part of the time. Set up your tent/hammock FIRST and then take the sleeping bag out of the dry bag inside the dry tent/hammock. Do the reverse in the morning if it is still raining or wet. If it is not raining and you are concerned about condensation from inside the tent, lay the bag on top of your tent in the sun while you are making breakfast and breaking camp.
Whatever you go with, get a liner of some kind. I like silk, but any will work, as long as it is not cotton. Liners are way, way easier to clean than sleeping bags, and your accumulated funk will go on one at night. If you get the thicker ones, they can add a decent amount to the bags temperature rating. Sea to Summit makes some that add something like 25 degrees.
And yeah, go to a store that knows what they are doing. They should have a torso measurer and weight bags to help properly fit the pack to you. REI employees are usually trained on how to do all that. For sleeping bags, get inside it in the store and move around. Insulation only works when it is not compressed, so keep that in mind. You want it to be close to your body without excessive air space, but not so close that it is compressed.
