Attic insulation - What thickness, vapor barrier foil?

You don't need a vapor barrier with concrete. I simply used a mix of EPS boards for the walkable area and wool for the areas you don't get to anyway. Lay the boards in two layers, offset crosswise. Then put OSB boards on top and done. It's significantly cheaper than ready-made products with walkable surfaces and I think the result is also better.

Exactly, with a concrete ceiling you can skip the vapor barrier foil. However, with loosely laid EPS boards I have experienced that they can shift over time with frequent use and cause thermal bridges, especially if flexible material is used around the edges. Also, the floor must be even, without bumps or cables, because EPS boards don't adapt at all. Otherwise, air pockets form in hollow spots, which renders the entire insulation useless. Other walkable insulation boards (wood fiber, rock wool) are much more tolerant there. EPS is of course the cheapest solution.

I would recommend wood fiber boards with a compressed surface in the walkable area. They can be walked on directly without an additional board and used as storage space. Then flexible wood fiber in the corners.

Alternatively, walkable rock wool boards with chipboard on top and flexible mineral wool in the corners.

As a top layer, I would recommend chipboard rather than OSB because chipboard is more vapor open; if room humidity gets into the attic or insulation through leaks (stairs, cable drillings, ventilation), it can evaporate better this way. You don't need dry screed from Fermacell or similar since you don't intend to live up there. 20mm chipboard is sufficient, or with wood fiber boards you don't need any.

Always work in two layers with insulation material; with flexible material in corners one layer is also ok.

If there is really no insulation present (are you sure there's not already screed + a few cm of mineral wool?), I would apply at least 200mm with WLG035, preferably 240mm. Don't hold back with flexible material in non-walked areas and rather use 300mm WLG035, it's very cheap. Wood fiber usually has WLG040, so use 10% more thickness here.

With concrete ceilings, insulating the attic stair hatch is useless because the surrounding concrete slab then forms a top-class thermal bridge. Here the stair must be airtight sealed and another insulated hatch/insulation hood must be installed on top in the insulation plane. For some stairs, this even needs to be placed on the insulation level. There are ready-made ones for sale or you can build one yourself from chipboard + insulation.