Well, normally, neither the ply nor the drywall should be in a tiled shower wall sandwich, so I wouldn't fee bad about tearing some of it out! You might find it in perfect shape, but you may also find that the drywall and ply is all moldy, especially down near the bottom.
A more typical wall would be vapor barrier on the studs lapped over the tiling flange of the tub, 1/2" cbu, then tile. There are some neat newer materials that are easier to install - Kerdiboard and Wediboard come to mind. These go directly on the studs and you tile to them directly.