Is the construction project feasible?

Oops, you are already quite far along :)

prepared for simple shell construction

I think screed and drywall will be included. But no floor and painting work. No wall tiles! No putty work... It is questionable whether, for example, the interior doors are included. "Sanitärrohinstallation" - as far as I know, all sanitary fixtures are missing! The construction service description gives you more information: everything that is not listed is also not included (in the price).

In addition, the windows will be white, the exterior plaster the simplest without colored nuances or even wood cladding.

And then it must be said that you have to pay real estate transfer tax on the land as well as on the purchase price - ok, you have to do that with a used property too!

Hello

in the worst case, no screed yet.
at best, screed is in. Do the floor work and wallpapering yourself.

I haven’t had a Bien-Zenker building specification on my desk for a long time; however, screed should be in place, otherwise the rough-in installation of electrical and sanitary systems makes little sense. So you should wait for the service specification.

or can that still be phrased more as "final finishing"?

Well, for example, a lot of filling work might be waiting to be done ... interior doors might be waiting to be installed (but that only works if tiling is included in the scope of work). "Final finishing" doesn’t sound like it only includes painting and flooring work as the only remaining option.

Rhenish greetings