Buying a single-family house plan - Architect for accompaniment?

: My gut feeling also tells me that too many distributed tasks are more likely to lead to problems,

Then listen to your gut ;)

What documents would belong to the planning to be purchased?

Dimensioned floor plans, elevations + sections.

In addition, the structural engineering costs for the object are certainly almost 10,000€; with my soil report he could do it for half the price.

I don’t see it; the planning of a city villa is not so complicated that structural engineering costs could add up to EUR 10,000. Listen to your gut, buy the planning, and arrange everything else on site! Rheinische Grüße