Hi kko. Horses do very well in the cold as long as they have shelter from the wind ect...I also give mine extra hay if the temps are 10 below zero or lower. That extra hay really provides extra "heat" for them through digestion. If you see one shivering you may need to blanket. I very rarely need to do this (I have on occasion for T, she's 20) but have when 10 - 20 below until temps rise.
Water will be extremely important. I bring mine warm water and they seem to know to drink quick before it cools. I have also in the past added salt to their feed to encourage water consumption. If you can use a water heater great.
It just fries me to see Walka standing in the paddock during a snowstorm or freezing rain storm! But they put themselves where they want, in or out.
If your horses wear shoes in the winter, you'll have to take precautions to prevent balling under their hooves. Mine are barefoot so no worries there. I've been told that if they wear shoes to rub the bottom with oil, or Vaseline or spray with PAM. I'm sure you'll get plenty of advice here.
Good luck with them this winter. I understand N Dakota gets cold