3. There was mention in a thread recently that regular plasterboard
shouldn't be used in a shower, is it really worth replacing some
existing plasterboard?

Personally, I'd just stick with the plasterboard. However, it is ESSENTIAL
to do a good tiling job if using a non-waterproof background. Use a top
quality adhesive suitable for swimming pools (not some value readymix
adhesive + grout ****e). Ensure (lift occasional tiles to check) that you
are fully filling behind the tile with no air gaps. Use a quality grout with
antifungal additives and narrowish joins. Really fill the gaps with the
grout. Seal the grout with Lithofin KF StainStop.

Christian.