What comes out
The bath, its panel, the taps and the waste all come out, and so do the tiles on at least the three surrounding walls. Expect to find one of three things behind them: sound plaster, blown plaster, or damp timber. The condition of the wall behind the old bath is the single biggest unknown in the job.
The waste and the fall
A tray waste sits lower than a bath waste and needs a continuous fall to the stack. On a suspended timber floor this usually means notching or re-routing between joists, which must be done within permitted limits, not simply cut where convenient. On concrete, a raised or slim-line tray keeps the fall without breaking up the slab.
Waterproofing is the job
Tiles and grout are not waterproof. A shower area needs a tanking system - primer, tape at the junctions, and two coats of membrane - taken up the full wet wall and out onto the floor. Skipping this is why so many five-year-old showers leak into the ceiling below.
Realistic extras
Common additions once the wall is open: replacing a perished shower valve, upgrading the extractor, replacing a section of floorboard, and boxing a soil pipe properly. Budgeting a contingency for these is more sensible than being surprised by them.