Hey Joe,

Here on the other side of the pond, we would approach that job, which
appears to be a complete redo with a complete and utter rip out. Get
down to basic structure on all 6 sides and start fresh. From there,
get everything as close to level as possible and rough systems
(plumbing and electrical) installed. In my experience, when you have
the rough framing and systems properly installed everything else is
goes in like butter. If you try and mess around retrofitting old work
to match up with new you end up with a half assed and more difficult
job.

Good Luck.