you should consult an engineer/architect for this to draw up the building plans to submit to the city. After all, you'll need to replumb, move electrical, ensure there are no venting issues, since it's attached to a shop make sure the fire rating of the wall is up to code. A $300 stud wall sounds like the least of your possible issues :)

Kids bedroom furniture (shelving, desk, bed surround) is amazing, LEGO!

Lounge doorway is 81cm wide with door on, and would be 82cm if could take off. Very old fittings that seem to have been painted over many times so getting door off may be an issue. Doorway height is 196cm Hallway width is 77.5cm and height wise no issue in hall - very high ceilings. The hallway width is probably the biggest problem. The only access to lounge is through this hallway door, and the width is only 77.5cm. The Next team were trying to twist it around but the sofa was just too big. They actually ended up accidentally putting a hole in my wall trying to do it!

I’ve got those in my garage and I think they could be upcycled. They did arrive in about 20 million pieces that in the end we had to get help to assemble them! I can’t remember what they cost. You may be able to get reasonably attractive ones from Ikea, as arc3d has shown, or maybe places like Dunelm and The Range, if they are still open online?