For the pre-heat tank, there are lots of indirect WH that you could use. Never saw an electric one in that configuration, though (doesn't mean they don't exist). The temperature gradient works better on a taller tank. I think you'd lose more capacity with a horizontal tank, since the colder bottom water would be closer...less gradient, and therefore, you'd be unable to get as much out before the hotter top water was cooled off.
A heat pump hybrid version may not be all that efficient if it gets cold there...might be fine if it is a mild climate. Tankless needs LOTS of amps. Not sure what you have that you installed a 400A service, but an electric tankless might need a lot of that, so it would depend on what your 'normal' load is.
Propane may be an option, if NG isn't available.