I live at altitude (a mile) in western SD. My hens spent last winter in an uninsulated metal tool shed (10x12). It was a super wet winter but no condensation after I added gable end vents. I only opened the doors on days when it was above 0* Fahrenheit, and they were closed many days, all day. I got a few spots of frostbite on some of the single comb girls which doesn’t seem to bother them, either at the time or now. The main cause of frostbite is humidity rather than cold, or so I’m told. If you haven’t yet bought your chickens, consider those with rose or pea combs.
Regarding your windows... make certain you don’t have your roosts positioned so as to cause a draft on the birds as they sleep. I use 2x4 boards, broad side up, so they can easily sit on their feet to keep their toes warm.
It sounds like you intend to use only part of your shed for your birds. Please keep in mind that chickens confined to their coop for long periods by inclement weather need more space. I wouldn’t plan for less than 10 sq ft per bird in my climate. Plus... if you order from a hatchery, be aware that they WILL send extras (who ime always survive at least the trip to your door and typically well beyond). If you want only 3-4, you’re better off to request your farm supply to special order the breeds you want or buy from a local breeder.