New slab extension on top of old slab.

Hey guys,

I need some advice! New to this slab game. Im leveling my timber floors and I have an old slab inside the rear (dodgy extension). The slab is stepped and in some spots too high. Im going to break these up and lower them but then I need to pour a new slab which will also cover these oldslab parts. What do I need to do?

Also concerned about rising damp.
In some spots the slab will be on the ground. Do I put down plastic before pouring? Do I rap my new bottom plates for the walls before building?
What is standard building practise today?

Cheers
:D