We would need to survey the property to give an accurate quotation to supply and install a structural  floor at the loft level, as we would need to check if you have internal load baring walls to confirm the size of the beams and joists required. With Reference to the head room, you must have 2m clear headroom above the staircase and above the loft landing at the top of the staircase. I think your best course of action would be to contact your local authority Building Control to confirm the exact amount of work required to make the room compliant. The reason i suggest this is that the regulations have changed over the last 20 years regarding fire regulations and insulation etc.