Shower as a whole would need to be gutted, which you already knew. As far as how much else needs to be demo'd that will need to be looked at by someone local as we do not know what wall condition, plumbing, drainage, etc. However I would wait to do the shower until you can gut and redo the entire bathroom. To do the shower properly you will be removing tile on the face and deck of the tub, at least a few rows. The wall may need to be rebuilt for cement board and tile as it will weigh a lot more. Plus the pan and the waterproofing, partial demo will be more intrusive than you think, if even feasible.