Now to the actual problem: We lack any sense of the necessary construction sum for this project. One would think that as a civil engineer I would have some home advantage, but I work in transportation construction and am therefore completely blind regarding the 300/400 cost groups. Nevertheless—everyone has a pain threshold: We would prefer not to spend more than €600,000 for the 200-500 groups. This would put us at just under €3,500/m2 of living space maximum. I actually calculated a sum of around €2,500/m2 as "desired" costs. I know that breaking it down to €/m2 prices is difficult, but it at least gives us a rough directional path in the upcoming talks with Gus/prefab house manufacturers. I would therefore be very interested to know which m2/€ prices you consider realistic for our construction project.
We are currently building very similarly to your plans in Bavaria. Our house is a bit smaller at 150 sqm. We are currently at about €630k all in. However, the plot only cost us €80k. So we have spent about €550k for the house including basement, kitchen, garage, garden landscaper (currently still estimated), fees, etc.