In my last place, I just strengthened the floor, added more insulation and extended the ring main and lighting circuit into the attic. I added a Wickes Means of Escape and a rope ladder just in case. I did all the roof joist insulation/vapour barrier and plasterboarding, as well as party wall soundproofing myself, and didn't remove any purlin supports so there were no structural changes. I used it as a cinema room with projector screen and speakers etc, but when it came to selling, I just told them to describe it as loft storage space. I had no problems with selling etc, and the new owners even wanted me to leave the pj and screen which I was happy to do, so for me it all worked out nicely.
Initially I was using a drop down ladder, but then used a removable fixed mini stair set up. It obstructed the landing a bit but added more convenience than it removed. It was easy to remove if necessary, and could be laid on the landing out of the way.