Heating is going to kill you. Insulate both under and around the pond and think about a greenhouse. An 8'x 12' pond built with two steps, one at 2'deep, and one at 3' deep with the deepest part being 4' deep will probably neep retaining walls and will contain almost 1900g but be deep enough to mitigate weather effects if it is protected in a plastic green house that could be removed in good weather but I would not risk it.
There are some great temperate weather fish from Europe and Eurasia such as sterlet and zander that you might look into. A scan of fishbase in the by country listings for place like poland or hungary can give you some ideas. Many of these fish are long lived and some will hand tame for feeding. American eels are also a pond choice. For a pond that size you will want 2 4250gph pumps and at least 120g worth of filter and maybe (2) 300g plant troughs. It only takes a few hours of power outage or one bad storm to wipe out an outside tropical tank in your area.
There are some great temperate weather fish from Europe and Eurasia such as sterlet and zander that you might look into. A scan of fishbase in the by country listings for place like poland or hungary can give you some ideas. Many of these fish are long lived and some will hand tame for feeding. American eels are also a pond choice. For a pond that size you will want 2 4250gph pumps and at least 120g worth of filter and maybe (2) 300g plant troughs. It only takes a few hours of power outage or one bad storm to wipe out an outside tropical tank in your area.