Building plot with already constructed basement from 2017

According to the sketch, I assume that by "Bodenplatte" you mean the already finished raw floor of the ground floor, which extends beyond the basemented area. Basically, I would advise using the cellar here instead of just demolishing it again for more desired ground floor space. Under "Informationen" in my profile, you can also see how to contact me by email (few other members handle this similarly). My reading tip for you would be

Demolish old basement approx. 15k new large basement 100k.

Whether it is worth it, you should decide yourself.

How is the condition of the basement? Already damp? And why was construction not continued? Were there already defects in the basement? 2017… that was 4! years ago!!! Why is it only being sold now?