Did you have any heating in the room?
Are the walls, roof/ceiling insulated?
Did you notice any condensation on the walls/ceiling/ windows(if any)?
Measure down from what you would like as your finished floor height, 400-450mm(depending on concrete depth you decide? 100mm should be fine.). That's 150mm of hardcore, whacker down, 50mm sand, whacker again, DPM, 100mm kingspan, then 100-150mm concrete. Place your mesh/rebar, lifted 50mm off the insulation(you can buy spacers).
As its an outbuilding/separate garage(different r-values to living space and not heated?) you can buy cheap polystyrene insulation instead of Kingspan, if you want?
Make sure dpm comes up the wall higher than the finished slab level. Trim it when concrete is dry.
You will need (for 3m x 5m room) approx 1.5m3 at 100mm depth and 2.25m3 at 150mm depth of concrete, quite a lot to mix. If you can drive to it, i would get it premixed.
Good plan with the ventilation.
One problem you may find, is that once the floor is done, the condensation moves to the next cold spot, the walls/ceiling?