Newly built single-family house - gas or air heat pump + photovoltaics + storage?

Since you have not yet stated which insulation standard you are aiming for, here is the reminder again:

The insulation has the greatest impact on heating costs, followed by the design of the underfloor heating, and lastly the heat generator.

Oh yes, the consultant said his services come to a total of €4,500, with half of that coming back through KfW - is that within reason?

Wow. What exactly is included in that? For me, it was €1,300 (BAfA/KfW covered €800). That included an on-site appointment with an inventory, consultation on what brings how much, and a room-by-room heating load calculation.

Oh yes, the consultant said his services amount to a total of €4,500, half of which is refunded by KfW - is that reasonable?

What exactly are his services? What does he do?

Wow. What is included there? For me, it was 1300,- (800,- covered by BAfA/KfW). This included an on-site appointment with inventory, consultation on what brings how much, and a room-by-room heating load calculation.

For a KFW house, construction supervision is included and then it usually always costs that much!

which insulation standard you aim for,

42.5 stone - N8 or N9, depending on what it costs.

The question was probably more about which insulation standard according to KfW you are aiming for, such as KfW55 or similar. A stone width alone does not say much.