Price-performance comparison: Which is the best prefabricated house provider?

Same here: 140sqm + basement all in under 300,000 (excluding additional construction costs)

but before I spend 50k on the basement, I would rather invest the money in a house one size bigger if possible

But that is not always possible, although there has already been an endless thread about that.
Long story short: your plot determines whether the house will be large or not. And if necessary, that means you have to move downward.

Which developer?

Schwörerhaus!

We then also went for the selection" fireplace, anthracite-colored windows, electrically controlled windows, staircase in anthracite, etc. but the base price was 10-15% below.

For your information: if additional companies are named and reviewed here, this topic must be moved to the moderated forum.