If you dig the pond floor down well below the frost line and place polystyrene between the wooden case and pond liner /fiberglass. Use bifenthrum treated polystyrene to stop worm & insect attack. Use several airstones (kept off the bottom to reduce bottom mulm) over winter to prevent surface ice although some ice fine. Do install bottom drains if possible.
Koiphen is the better online koi resource. They are fanatical about their Koi and most of their members are in the northern hemispheres like you.