In a nutshell I want to create something that is realistic looking (berlin flat and others) but where players can build their entire house/ business like the sims franchise.
I’m not a complete novice with ue4 but some pointers and info on such a project would be great.
However, I can only work with blueprints.
Anything at all is appreciated 
Sounds like your foundation will be building and decorating your structure, with various sim systems that exist within that structure. So I would focus on creating some kind of house creation editor, and add features from there.
The way you add walls and whatnot in the Sims reminds me of working with the splines in construction script the way you can just drag things around and they automatically grow to fill the gap. You could probably use this, or create your own simple setup with blueprint. Of course, I don’t remember being able to build curved foundations in the Sims, so maybe starting with splines isn’t such a bad idea.
Once you get the building editor to prototype, it should be pretty simple adding systems such as objects that boost your Sim’s stats when they interact or are near them.
The harder part at that point will probably be the AI and interweaving all the rest of the systems together.