we had great luck with no heat here in cold new england last winter.
besides deep litter and good air circulation, i rigged a "vestibule" to keep the wind from coming in the pop-door, and i built a cookie tin water heater.
no additional heat in the coop, and i only had one day with frozen eggs (it was because we were away for a day and didn't collect the previous days eggs!)
we had no frostbite, and no illness.
good luck!