Work on basement ceiling insulation with stone wool

Better 24cm something. The cheapest is cellulose between the rafters and then cross battens with even more cellulose and on top rough boarding / rafter sheathing. For wooden beam ceilings, don't forget the vapor barrier on the bottom and work on making the connections AIR-TIGHT!

I just wanted to insulate the floor of the attic including the folding staircase. Thought that would be the simplest and cheapest option, right?

Yes exactly, nevertheless the transition from living space to insulation in the attic must be airtight or sealed. Otherwise, the insulation will not help and will become damp due to condensation. Whether the sealing is highly vapor-retardant or not is not that important in ventilated attics. Much more important is that there are no leaks (e.g. ceilings with small-part wooden paneling, side spaces). Insulating and sealing the attic hatch is also a sensible measure.

So 10cm PUR usually costs 35 euros per sqm? 2nd choice costs about 20 euros per sqm

sorry, I am a complete beginner here. I have to ask a stupid question again: So I put a vapor barrier on the floor of the attic, then put a thick insulation material on top and that's it?

What kind of ceiling do you have and how do you want to use the attic? Does the insulation need to be walkable?