Old plaster came away, render looks solid and flat what do i need to seal it with prior to tiling?

[Qwerty]

IMO dry lining the whole room with plasterboard and tanking only the shower area is more cost effective than tanking boards and tanking tape/ paste for the joints.

Don't get me wrong, tanking boards are superior all day long, but when cost is a consideration then dry lining the whole room is cheaper.