You will likely need several coats for different temperatures and weather patterns (rain), but if you find the kind with a water resistant shell and a zip out lining, that can count as two (and you can layer underneath with under armor or Wintersilks). Lands End, LL Bean and others make those kinds of coats.

I would avoid down coats with that silky nylon shell as it can snag and not wear well. But I have a decades old down coat from the Company store that is supposed to be warm down to something like -20 degrees.

A longer length and a hood are helpful to increase warmth.