How much house is realistic for us?

I would still have doubts that it will work. But why one house on one plot, and another on the other? Shouldn't the choice of plot and house be independent?

The prefab house does not fit on the one plot because it has to be built in an L shape.

Which area are we talking about? The prices seem very (too) low to me. Kitchen and outdoor facilities? Never! There alone at least 20k€ is gone. Regarding the plot - without knowing or having seen it - I would rather tend towards the edge of the forest.

Well, the architect gives a price guarantee for his calculation. This is also contractually agreed upon.

Is the region between Hannover and Braunschweig

Then I would have it broken down exactly what is included and at what price/quality. Otherwise, later on, it will be said "yes, that was not included in the offer."