Here's my 10 cents worth. Silver soldering(brazing) copper to copper. Use 15% silver brazing rod and you dont need flux just make sure pipe is clean (use steel wool or equivalent scourer).
Air to water heat pumps: These are quite efficient but careful on the brand. They are basically an airconditioner on reverse cycle but cheaper ones don't last. they have an EER (energy efficiency ratio of 3 and above which means roughly 1 k/w of electricity for 3 k/w of heating. https://www.daikin.com/products/ac/lineup/heat_pump/
You really need to heat the water to 70°C to disinfect the water. Legionella and most other bacteria can survive in 60°C (140°F) water. Legionnaire's Disease Outbreak Warning In North Thailand - Thailand News - Thailand Visa Forum by Thai Visa | The Nation
You need a thermostatic mixing valve to lower the temperature on the outlet of the tank which mixes cold water with 70°C water which lowers the water temperature to prevent scalding.
An alternative is to use your existing airconditioning unit to make hot water, although it will only work while the A/C is running. https://techcrunch.com/2011/06/12/ai...-water-heater/
I'm lazy so I would just have a solar electric panel installed. then I can just use a conventional electric hot water storage unit.