don't worry about heating it. The initial price isn't too expensive, but the cost of running would be very high. When you heat the pond, you have to heat the water, when you us a space heater, you have to heat the water and ambient air. At any decent pond retailer you can get a floating deicer (some even run off batteries) that keep a hole open to allow for gas exchange.
The only time you would need a heater is if you find your pond is freezing all the way down. A good safe bet for most north america is 5 feet, but you could probably get away with 4. As long as you don't turn them into Koi-Cicles, they will live.
When water temps get that low, koi start to hibernate and you won't have to feed them (what little nutrition they need they can get from micros floating in the water). The hole just allows for gas exchange. If you want overkill, you can added a bubbler (just make sure the line doesn't freeze) so drive off excess CO2.