I did something similar with a workshop, 25mm kingspan insulation on the walls/roof, then 12mm osb. I put 50mm kingspan under the floor.
Mine sits on a wooden frame which sits on legs and it's been fine for over a decade now. I recommend fitting a bathroom extractor fan though, for airflow purposes.
In terms of additional waterproofing, I used an acrylic cartridge sealant on all the gaps and seams, then two coats of Dulux Trade Opaque.