The easy answer is to set your intention for the coat (commuting to work and general wear, snow sports, working outside), set a budget and look for some decent coats and try them on in-person. There typically aren’t temperature ratings because temperature is extremely personal. I usually don’t wear a winter coat until it’s well below freezing.

For a climate that is below freezing for multiple days or weeks on end, but not an extremely harsh climate, I’d go for a shell and liner type coat. This is typically a water-resistant outer coat with little or no insulation and a fleece jacket that zips inside. The lowest I’d be willing to go to insure that you still get good use out of it is Columbia. Other brands that will be useful for many years are Obermeyer, Patagonia, or nearly any brand carried by REI. Ski/snowboard equipment like Burton is decent and Lands End or LLBean in the US is also a decent bet. REI and Patagonia have used marketplaces online if you know your size and what you want. I personally would not go to a secondhand store unless it was high end consignment or very well taken care of.

Avoid Canada Goose or luxury brands that don’t specialize in outerwear.