Just remember that herons and raccoons love eating fish.....especially expensive koi. Coons will really tear up a pond. Remedy, an electric fence for the coons.
Ponds are fun. Goldfish and koi do not like to overwinter outside. For them it is a matter of surviving without eating for many months. You must keep a heater at some spot to keep the ice open. This allows for gaseous exchange. Without it, your fish will not survive.
PM me wth any questions. I have had an 8' x 16' pond for over 25 years with no problems. The best part are the water lilies.