If 150 is the floor area, that is a big bathroom, but if that includes the walls...wow!
A toilet must have 18" from center to wall on EACH side.
If we assume the original shower was not well done, you may find a lot of water damage when you get into the walls.
Replacing the valve is not a big deal for a plumber once the walls are opened up.
If this bathroom was bootlegged in, we have to wonder about whether the drain and vent are proper.
All these things will have to be made right when you get the permits for the remodel.