I've done this in my mid-50ies hous, and it is not particular difficult, but needs to be done correctly.
There's probably no harm doubling up the timber joists (the ones in my house where only 7 inch), or at least add bracing, to minimise bounce if not already planned.
Use cement board then instead of plywood as the floor, and something like the [broken link removed]as the shower element.
Please note that installing the jacko board requires pretty much building a floor between the floor joists, so make sure your builders reads the installation manual and does it by the book, and then some. Double-check every piece he did on this, as you don't want things breaking.
Once the floor is stable, it needs to be lagged - either using something like the
DITRA membranes (not cheap) or at minimum with a proper [broken link removed].
I am now into one year of usage and so far no problems - no leak, no cracks in tiles or in the grout. really love the wet-room type shower so gets a lot of use!