Hi swix112,
€uro is right when he advises you to consult an independent expert. There are different ways to find one. Either an energy consultant through the DENA (German Energy Agency) or the BAFA (Federal Office for Economic Affairs and Export Control) or a specialist engineer for building services engineering from your area.
The KfW application is basically unsuitable for using data because it only contains a few results of the energy saving ordinance proof. However, if you want to develop an energetically sensible overall concept for a property, it is necessary to align it not only with the framework conditions from the location but also with your user behavior.
This is not done by the energy saving ordinance proof, even less so in a GU/GÜ project, where the turnkey house builder has a "standard proof" for the house model X. Actually, an adjustment based on your wishes and the resulting planning changes is required.
I would also not let myself be "driven" by a €2,500 price difference, because – even if it is in the private sector – you also have to consider the profitability of the investment. After all, the system will be in operation with you for 15 – 20 years, which is quite a long time.