I'd start by having a few tradespeople come in to quote for shower waterproofing and plumbing that you need removed and relocated. You can potentially at that time enquire as to their requirements for flooring and levelling.
Self-levelling the floor and the render you can do yourself, but consideration will need to be made about the final levels and what type of floor the shower will have.
A Kaboodle 2400 x 600 x 36mm Hevea Timber Benchtop is suitable to be used as a vanity benchtop. However, as with kitchen benchtops, they must be coated with a sealant suitable to the environment they are in. The sealant must be applied on all surfaces including underneath and inside all the cut-outs for taps and the sink. Once the sealant has been applied the benchtop will be water-resistant but not waterproof so any spills need to be cleaned up as soon as possible.
A suitable sealant for a bathroom vanity top would be Cabots Cabothane clear.
For the many different tasks in a bathroom renovation, you should find these step-by-step tutorials helpful:
I've built one bathroom and am finishing renovations on another. I found getting my plumber in to do the under-house work really laid a good basis for me to install the shower and sink myself. That is, he put the pipes in the walls and floor and connected everything up to our rainwater tank, as we are not on mains. His willingness to offer some guidance later was invaluable when I was preparing the walls for tiling around the taps. I suggest that you engage a good plumber to check all the pipes before they go behind the walls, as well as an electrician to check or install any wiring.
Compared to the technicalities of getting the water and electricity into the bathroom (legally and safely), the rest is a doddle. I built a timber framework and attached cement sheeting to that, but I've seen some handyman videos where the cement framework has been attached directly to sound brick. But I prefer the certainty of nails and screws to glue. Tiles are very heavy, and I didn't want my cement sheets coming away from the wall.
There are a few different waterproof backing sheets for tiles and all of the makers provide excellent guides for installing backing sheets and then waterproofing them further for tiling. Bunnings has really simple videos and Youtube is full of advice. - blb48
I suggest making a list of the items that you would like to put into your new bathroom. I recommend considering if you will need the services of a plumber and electrician. By making this list it will allow you to see which parts you can do yourself. It will also give you a rough guide on how much the renovation will cost.
When placing an item in your list, it's important to consider its size and shape and the direction it will be facing in the bathroom. Will there be enough room to walk through once you've placed the item in the bathroom. There are several ways to proceed: