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Englefield Milano square shower install with acrylic tray

I am looking for somebody to come and quote a shower install for a Englefield Milano --- shower. The unit has an acrylic tray and will be a tiled wall shower. This link shows the shower: --- The bathroom is currently bare, with H3 ply subfloor and aquastop drywall wall lining. I know I will need to install cement board to the floor and leveling compound over the heating element before I waterproof (prior to tiling). What I am unsure of is whether the drywall needs to be removed and replaced with cement board prior to tiling and waterproofing, or if it is possible to install cement board over the drywall, and then waterproof. If I could get a quote for the install, that would be great. Also - I have 15L of waterproofing membrane that will need to be used.