You are getting great advice here from these guys ... Give us a layout drawing (doesn't have to be professional) of the space and how you envision the finished bathroom, with all items in place. Another drawing with the adjoining rooms and location of water, electrical and sewer would also be helpful.
Why?
This defines the project which, in turn, defines the parts list. Honestly, the parts come in stages depending on the space requirements. For example, you have a storage space ... but you need a "wet wall" framed with a 2x6 to allow for plumbing of supplies, drains and vents. The distance to the sewer defines the amount of main drain pipe and fitting types. The condition of the floor and your eventual floor surface defines whether you need to add Hardie board or plywood. Is your exterior wall properly insulated? It has to be or your bathroom will be cold. Do you have HVAC access and where will the vents go in the new space?
All this is good. Construction is a process you need to look at in phases then step back and see if you left anything out. Hang in there ... working off a slab and no attic means cutting into the walls to route water and electrical, not to mention sewer.