I agree with the posts above but would just add that it would be worth looking at gas heating especially if you have gas central heating in your house. If you decide to run an electric heater of more than 2KW then the costs of setting up electric (as you will need a dedicated supply IMO off a consumer board which you may or may not already have available) compared to gas are very similar & gas will be cheaper to run. Several people run their koi heating off their house boiler & I believe there are some threads on here on how to set it up. Not knocking electric heating just adding some food for thought.
6,250 gallons, block & f/glass, aerated b/d, skimmer, nexus 300 & nexus 200, sand filter, 2×55 watt uv, 2x40l air pump, gas heating, 4 pod purifier, autofeeder