I'm going to be moving to Northern Sweden this year and am working on getting geared up in the US being that most everything will be cheaper here. I expect to be doing some winter camping in bivouacs, snow caves (maybe), and/or tents and the temperatures could get down to between 0ยบF and -20ยบF while I'm out there. It's been recommended to me to get a bag with at least a 0ยบF rating, with preference being an extreme temperature of at least -20ยบF if not -30ยบF preferably in synthetic being that wet down could be life-threatening out there.
The bags on the table;
1. Mountain Hardwear Lamina -30 – cheapest, synthetic fill, coldest advertised rating.
2. Mountain Hardwear Phnatom 0 (Women's because of my build and extra insulation in torso and feet) – Could I comfortably/reasonable get below 0ยบF with a VBL and bivy sack with this set up?
3. Big Agnes Hahns Peak SL -20 – 3.5 lbs, cheaper than the Wraith, 800 fill down, yet no insulation on the bottom (comes from pad only) and only DWR on the outer shell.
I know everyone recommends Western Mountaineering or Feathered Friend etc etc, but of these bags, which would be the best bet, and what precautions should be taken to keep it dry while in use? Would it be sufficient to go with a bag with a rating of 0ยบF or somewhere in the middle and rely on a bivy and VBL for extra warmth down to -20? Lots to consider in such an investment, so sorry for the slew of questions, but I appreciate advice from the experienced.

