What material costs should be expected for interior construction?

My assumption is based on several statements from different construction companies, who always advise against air-to-water heat pumps. They would only install it at the customer's request, but it would probably not be profitable, especially because we can have harsh winters. That is why they advise against air-to-water heat pumps. The statement was to wait 10-15 years until the system is amortized, and then see what has established itself on the heating market. It might be worthwhile to retrofit afterwards.

A gas connection to the public network would be omitted because in the small village where we want to build, there is no network available; therefore, an underground tank will be buried, which is already included in the total offer price.

KFW 55 was once a topic for us, but after further research, we concluded that it is not profitable, as the extra cost is only amortized after more than 30 years through savings.

For an underground tank, I would then go for the air-water heat pump... but that's more of a personal preference. I wouldn't want to track the gas price all year or be annoyed that I refueled too expensively.