Heilbronn. Unfortunately, it has been a reality here for quite some time. Towards Stuttgart, it gets significantly more expensive :D fun...
Well, I consider 640k to be realistic. We built 3 years ago with 165m² including basement + double garage + photovoltaic with 16.5kWp - otherwise exactly the equipment you mentioned. Including complete outdoor facilities for a 550sqm plot (not all but a lot secured with L stones). We ended up somewhere just over 500k.
Since you are building comparatively much more stripped down, I could imagine that the price is indeed realistic for today’s conditions.
I would definitely write to the local solid builders (in our case, that was ultimately the best offer).