Couple of thoughts to add to the other good info here:
1. I have an Osprey Aether 85L. It's gigantic, not too heavy (4.5 lbs?), but I find it pretty uncomfortable and sort of hate it. If all my gear wasn't uber-cheapskate stuff, I think a 70L would probably be ok. Maybe even smaller if you're rich. Reasonable cost, reasonable size/weight, works in harsh conditions. Pick any two.
2. Sleeping bag: North Face Dark Star, rated -40, synthetic. Got it for $225 or something about 8 yrs ago; I was poor and knew it would work for both Baxter and Denali, which were both on my short list at the time. I've never been cold in this sleeping bag, and I hate this sleeping bag. I really can't wait to replace it. Stuffing it into the absolute largest compression sack I've ever been able to find is an epic workout every morning. Then I sit on it and rail on the straps to compress it with all my strength. Then I take the roughly washing-machine-size (and weight) package and shove it into my gigantic pack, and it consumes most of the space therein. There is no exaggeration here. My recommendation: get a down bag from a reputable company rated to -20 F. Expect to pay $700-$900. Start making peace with that expense now. Or go quilt or whatever, which I know nothing about.
3. I have a Thermarest Z-lite Sol CCF pad, and on top of that I use an inflatable (I really love my Sea to Summit dual chamber Comfort Plus - on the heavy side, but I sleep GREAT). The combo is very warm.
4. My tent is a Nemo Tenshi, I've had it for a couple years and love it. 5.5 lbs, bomber, and has a vestibule one can cook in comfortably. This is my one expensive piece of backpacking gear. I also want to get something floorless for my below-treeline trips but haven't pulled that trigger yet.
5. I use a MSR Reactor and love it. Have we had an epic canister-vs-liquid fuel battle yet this year?
Overnight solo weight is probably 45 lbs. With a partner, more like 35. If it's a presi-traverse backpack, probably more like 50 (even with a partner - crampons/axe, more food, more fuel). In my opinion/experience, it's very hard to get below 35 lbs for a solo overnighter planned near 4k' elevation. Assuming you want to keep a reasonable safety margin, you need to be either very experienced with gear that is absolutely dialed (in which case you wouldn't be asking us for advice), or you need to have a considerable cash flow available to blow on hiking. I personally don't like being cold, and I'm basically a scaredy cat, and I have kids with 529s to fund. So my pack weight is probably on the high side. I think I could reasonably decrease the weight of my down coat (an old 725 FP EMS belay parka), sleeping bag, pack, and some of my clothing - could probably lose 5 lbs, but that would cost in the range of $3k. It's hard to justify that cost when my stuff 'works.'