Hello Kati,
Let's start again from the beginning; maybe this will help you take the next right steps. First of all, your problem or questions cannot be solved here, so to speak, from a distance!
Basically, it is still the case that people do not build very often in their lives, and in Germany we are more location movers than in other countries. Real estate has also played and continues to play an important role as retirement provision and is thus an investment. At the same time, the importance of operating costs, essentially energy costs, is increasing and coming into focus. These will significantly influence the value retention of the property today and in the future.
Against this background, it is important to know that we are currently transitioning in terms of legislation from the Energy Saving Ordinance 2009 to the Energy Saving Ordinance 2012. The Energy Saving Ordinance 2012 represents a further reduction of the currently applicable energy level by 30%. Therefore, you should definitely plan and build now according to the Energy Saving Ordinance 2012 to avoid having an "old hat" next year.
If you build a property according to the Energy Saving Ordinance 2012, you can take advantage of attractive funding or low-interest loans from Kfw, at least for parts of the loan. A funding or energy consultant can provide information about this. You can find one at DENA or DEN e.V.
An energy consultant should develop a concept for your property together with your planner or developer that meets the framework conditions described by €uro. Essential are your personal user requirements, the legislation, and those triggered by the location.
To return to one of your questions, basically, a gas condensing boiler system is possible and sensible in an Energy Saving Ordinance 2012 or Kfw 70 house. It is characterized by low investment costs, high supply security, and environmental friendliness. The system technology is considerably cheaper than, for example, an air heat pump; a factor of 1 to 3 is realistic.
The environmental balance of gas is significantly better than electricity due to losses in electricity generation and transport (keyword primary energy factor). Biogas is gaining importance; it can be used in purified form as an alternative to conventional fossil gas or obtained from the energy supplier.
Due to the energy transition, there will be further price increases; with electricity, studies predict quadrupling in the coming years. This is expected to be lower for other energy carriers. This shows that even if, from the perspective of many, solar water heating and heating support currently do not make sense economically, they can contribute significantly in the long term to cushioning the price development.
In summary: Find an energy consultant! Together with the planner, combine wishes, requirements, budget, location conditions, and legislation into a concept and plan and realize the property accordingly. This includes, besides the topic described above, questions of age-appropriate building or the question of what happens if the children move out? How can a single-family house be easily converted into a two-family house?
Regards Erik