somehow I thought of something else with the calculations (what pays off etc) ops:
You can already see that gas is very good, as others have also written. Usually easy to install and not too expensive. I would just do it and not think too much about it. If it has to be even better, then you can consider a heat pump and calculate more precisely, but the savings won’t be huge, if there are any at all.
A storage tank costs max €500 plus labor and a few pipes. With 4 people, it is worth it compared to an instantaneous water heater.
If you want to calculate more: you have to estimate your consumption, number of people, price increases, etc. yourself. Your craftsmen can tell you exactly how much the investment will cost.
Photovoltaic systems are supported in the sense that you can sell the self-generated electricity at a fixed price for 20 years. The system pays off after about 12 years, depending on the purchase price. Even financing with a loan makes sense. The bigger the system, the higher your profit and the more electricity is available for your own use in the dark winter. Therefore, do not build a 5 kW mini system if there is more space and don't be sold a power storage.
And consider photovoltaics independently of the heating system.