I tried several of the packs you see advertised but none of them can come close to the comfort of hauling heavy loads as the Kifaru does for me. Buy once cry once would be my advice.
Id go with either the Kifaru puffy or the First Lite Uncompahgre puffy. They both breath well and are very water resistant and very warm. You don't need much for your legs just some wool underpants and a good set of pants like kuiu attack, first lite corrugate guide. The temps in the mtn's...
I use First Lite products and have been very pleased.Use the Uncompahgre puffy for my sitting and glassing jacket. I use their wool shirts and wool under wear with the Kanab pants. also the halstead fleece for a midlayer . This set up is good for temps down to the upper teens . At least for me...