well has it's only taking foot traffic and bikes I would have thought that 4" of concrete would be fine, if you don't mind the finished floor level being a bit higher than it is now, break up what you have, blind it off with some sharp sand (make sure theres no sharp bits sticking out) put your DPM over that and then the concrete, not sure about insulating the floor starts to get a bit expensive then, I would get hold of a nice bit of carpet or thick rubber mats or there is some stuff that you can paint/pour on, self leveling I think but don't know how it reacts with oil and the like