mh, a lot of questions remain unanswered.
But I will address the point BP. Earthworks are necessary for that as well. Let's assume 25-30cm BP, below that comes gravel. Usually around 40-50cm. But depending on the soil condition, it can also be more. Maybe insulation under the BP? That makes approximately 200-250m³. It may be possible to store topsoil on the side (20cm?), then maybe 170m³ or more remain. If the normal soil can be re-used, good, if not, it will be more again due to the construction pit / slope, certainly another 80m³.
What about iron and statics? Are those also "at a special price"?
Is a soil report already available? And keep in mind: your BP is really huge!
I don't consider 10K feasible even with own work and special prices. Read one or more threads matching your floor plan ideas, for example:
https://www.hausbau-forum.de/threads/leitfaden-zur-hausplanung-auf-dem-Grundstück.30891/