If you are going to use a water heater, I would not go that route.
Water heaters are not really that efficient for heating.
Even though they may have what appears to be a high rating - this rating only relates to water heating for showers, dishes, washing machine, etc.
It's a different rating system than boilers (which I recommend).
The problem with using a water heater for both heating and domestic hot water is that you run the risk of bacteria in the water in the non-heating season which can lead to Legionairre's disease.
If this is a long-term investment I would bite the bullet and use a boiler.
Viessmann has a gas boiler (it's called Vitodens) that has a 95% efficiency rating.
