Last year I watched two crows take a Red Tail out of the air and kill it. I was amazed. I've lived on the prairie for 50 years but have never seen that before. Seeing crows and blackbirds tormenting a hawk is a daily occurrence but I had never seen a kill.
It sounds like you have a difficult problem on your hand with no easy solution.
Unfortunately even the best dog is not going to be 100% effective if you have a predator living close to you. Free ranging chickens like to wander and the dog would have to be on duty with all of them at all times...
Raccoons are clever and extremely adept with their paws. I left a bag of dog food in a car with the window open about four inches. They reached in and rolled the window down enough to get inside the car. Coops need to be well coon proofed.
Much will depend on the size of your coop, the size of your windows, and whether or not you plan to let the chickens out during the day. A coop in direct sunlight can become very warm during the summer. That is much more if a health concern than the winter cold.
My present coop is small with...