Wet room/shower-trap/tiled floor- Advice encouragement please.

We are installing- well, that's just me- a new shower-area which should be accessible for a 'wet' wheelchair. I have found an earlier mailing, now some 2 years old by Peter Taylor but would appreciate some up-to date advice.
We have a concrete floor with engineering bricks as a face-house some 100 years old. I intend to put ceramic tiles 33x33 cm down as the surface.
I will have to cut a channel into the floor to take the new waste pipe to the new shower outlet, probably a Marley DF225 (90,- ouch), a horizontal outlet for a tiled floor.
I'm not yet sure that the floor is free of damp as thew bath is still in place which I'll rip out tomorrow. I'll test for damp by taping a 100x100 cm piece of plastic to the floor and hope no condensation builds up.
If the floor is dry can I just tile straight on using a water resistant mix, and grout with a water proof grout? I intend to lay the tiles wall to wall and then place 15cm cuts of tile to the vertical of the floor sealing the lot in. The wall tiles will sit on the 15cm of vertical floor tiles.
Should I: prime the floor seal the floor use self levelling compound do something else
When having cut the channel for the drain pipe can I just lay this in and cement over or should it lie in a bed of sand or sand cement mix?
Can I just cement the Marley trap into the hole or should I protect the thing in some way?
I have read postings talking about using all sorts of different materials but ceramic tiles but we- the domestic authority that is- quite like the tiles that look like slate with a rough surface which will aid our ballance.
We would like a door rather than a curtain but on a smallish budget having one made may not be realistic. Could one engineer a recessed rail in a gap between the tiles for a door to roll on?
Any tips, encouragement are very welcome. I'm also looking to install a thermo mixer to go with the combi but that will be another post.
Thanks folks; happy DIY-ing

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