Good weather proofing, zippers to vent moisture, made for hiking snowy wet mountains.

Wool base layer: https://www.smartwool.com/en-us/men/base-layers/tops/mens-classic-thermal-merino-base-layer-crew/SW016349.html?dwvar_SW016349_color=001
Wool blend hoodie: https://chromeindustries.com/products/mens-merino-cobra-3-0-hoodie
Rain shell: https://www.outdoorresearch.com/collections/mens-rain-jackets/products/mens-stratoburst-stretch-rain-jacket-300893

I haven’t tested it in a torrential downpour as I usually wear dedicated rain gear for that but for the cold it’s amazing

It’s expensive as most BIFL stuff is but especially if you are in the U.S. I 100% recommend

You can get a field jacket from any military surplus shop in the US, or from flea markets like Peddler’s Mall.

The particular coat I bought was the Women’s Down With It Parka, but I don’t know what the best options for men are.

The waterproof coat I bought was a collaboration between Hawk & Co and Goodyear that was a limited edition run, I found it at a Macy’s. It has survived many years of rugged outdoor use, e.g. bicycle commuting >1000 miles. Unfortunately I found it through luck and it has served me better than I ever expected, but it’s not terribly helpful advice to tell someone to go hunting for a limited edition coat that was sold probably near a decade ago. So hopefully someone else will have advice on a waterproof outer coat suggestion - the ones I looked at recently at REI were not promising, with lots of people complaining the waterproofing was inconsistent or unreliable. I suspect Tractor Supply might have decent heavy-duty, waterproof rain-coats that could work?

EDIT: I will also say, I went to my local Macy’s towards the end of summer and found some heavily discounted winter coats, the one I bought was Steve Madden brand, and even though it’s not as high quality and doesn’t have the benefits of the repair and return policies Patagonia has, so far this season it has kept me as warm as and held up as well as the used Patagonia coat I bought. I don’t know how it will go in the long-term, but it was a much more affordable option (I spent maybe more like $120 compared to $240, so around half the cost), and so far it has worked just as well.

EDIT2: oh and I meant to say, I’ve worn these coats in -20F temps and windchills colder than that, and they have served well. When it’s that cold I sometimes wear wool baselayers underneath the coat.