We have a similar set up though the water is pre-heated through a desuperheater on our geothermal system. We went with tankless hot water heaters...it only needs to burn fuel to heat the water from the input temp to the desired output temp. And it only heats the water on demand.

From my research, most tankless does not play nicely with pre-heated so do your homework on the model capability for that route. I personally dont think tankless are the right fit for most projects.

Sounds like a great candidate for a heat pump water heater depending on heater's location or available space. My next choice would be to cut out the gas and go with a Rheem Marathon.

Because I didn't come up with a definite plan I could lobby for fast enough, my parents are going with a conventional gas heater to go with the solar.

Personally, I liked the idea of tankless.

BrianKnight, out of curiosity, what would you need to know? It's a short distance away from the furnace, near a bulkhead, in an area with sandy soil. Fare amount of basement space.

If its in a basement, I would absolutely go heat pump water heater.