I would support the recommendations for insulation, floor, walls and roof. You have only one chance to install it and that is at the construction stage and the insulation should be as efficient and as thick as you can stretch to. The most efficient insulation is Celotex much better than polystyrene or rock wool but also more expensive. I built a shed in my garden for a darkroom and just wish I had put in more insulation but it is not possible now.
The extra £150 spent now on DPM+insulation will make a huge difference to comfort levels over the coming years.
Simple is good. Tar paper, plywood, caulk, and paint should do it. Never mind the cabinetry joints, just butt things up and screw it together w/ plenty of glue and caulk. Then roller paint it. I live in Florida, so there's no need for insulation. However, there's a need for a/c, which is why my darkroom is still the bathroom.
One trick I learned from having numerous contractors fleece me was to use good long screws instead of nails. Much sturdier, and you can put things together quickly w/ a power drill that has a screwdriver bit chucked up in it. Makes it a lot easier to take apart and set up someplace else if you move too.