Recommendations for Bathroom Walls (and Floors)

[ASTFA]

Hi guys, I was looking to completely renovate my new small-ish bathroom (1.63m wide x 2.52m long), but even with the assistance from search engine results I was still a little confused about which materials are recommended to use as wall tile substrate(s) and the relevant adhesive/mortar.

The type of tiles that I'll be using on the wall is what I've heard called a "doctor's tile", a plain white 150mm by 150mm x 4-5mm ceramic tile.

Here are some of the queries that I have in mind:

Wall Related:

(1) My uncertainty begins with the backing material that I should use for the walls. To reduce costs I am considering to use 6mm XPS boards for the two walls that comes into direct contact with the showering corner of a shower bath. Would the boards be strong enough to hold and resist moisture getting through in the long term? What sort of adhesive should I use and what should I use between the gaps?

(1a) What is recommended (cost effective wise) for bathroom walls that are likely to come into direct contact with shower water?

(2) How durable would ordinary plasterboards be on the other two sides that do not (or should not) be in direct contact with shower water? (This was recommended by the bathroom renovator I'm hiring labour only for the job)

Floor Related:

(3) My floor can be completed with 5x HardiBacker boards (1200mm x 800mm x 6mm, which I plan to use), what sort of adhesives should I use? Are there adhesives that are supposedly waterproof or water resistant themselves?

As far as I am aware that for the bathroom floors the boards needs to be "glued and screwed" (as recommended by one of the threads here), and I plan to use fibatape to seal around the edges as recommended by the official HardiBacker videos.

Finally

(4) Is there a type of grout recommended for bathrooms on the more economic end of the scale?

(5) Just to confirm, is tanking necessary for my bathroom?

(6) I've seen here and there that tiling underneath the bath tub is not necessary on the web, is this really the case (assuming the bath tub is properly installed and sealed up)? Would there be any harm tiling underneath the tub as well?

If relevant, my preference is non-fancy, cost-effective economical stuff (stuff that works of course).

The property was built in the '40s - '60s, I've taken a few of the tiles off and think it's some sort of a plaster underneath (brown powdery ?plaster material between two grey layers of plaster), not sure what's behind the plaster though. I'll (try to) update with some pictures tomorrow.

Thanks in advance, I'd really appreciate the advice.

 

[ASTFA]

* I intend to have the bath tub installed on the wall with the window with drains on the left side.

Update: I went over today to start taking some of the tiles off and found that the tiles were placed on a dry wall. Most of the dry wall seemed intact with some cracks, except for part of the bottom areas where bits have fallen off.

Given the condition of the dry wall is it worth repairing it and use it as the backing for the two walls that should not come into direct contact with the shower?

Please help, thanks.