Replacement Shower Project

I want to replace the small shower I have in my ensuite. I will be changing the lot, shower tray, shower, door, tiling etc. It is currently in a recess so no cubicle required, just the front door.

1) If I remove all the tiles, can I just tile onto plaster walls or do I need to put some kind of waterproof layer in between?
2) I currently have a cheap acrylic raised shower tray. Can I change that for a nice stone or resin one, and if so, can they be raised in the same was so I can use the same waste system?
3) We have a very poor combi boiler. I have had to turn the cold water inlet into the boiler down so that it can heat the water properly. If I open it up fully, it can\\\'t heat properly. In this scenario, is a water pump for the shower pointless?

Many thanks for any advice you can offer.

) We have a very poor combi boiler. I have had to turn the cold water inlet into the boiler down so that it can heat the water properly. If I open it up fully, it can\\\'t heat properly. In this scenario, is a water pump for the shower pointless?

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you cant pump off a combi

I just tile straight on to the old tiles or the plaster.
Put the new tray on to a piece of 3/4inch marine ply and bed it in with a weak mix of cement, say 2 or 3 to one

Please see forum rule 17