Couple of points not. Be mindful of head heights of your new stair. For both going up and potential clashes with roof rafters (depending of pitch direction of roof) and below where you place it relative to the existing stair. (Do not use spiral stairs!!!) Fire regs You will also be essentially turning your house into a three story dwelling. This opens up a whole new category of fire regulations within Approved document B. Which affects the escape routes and requires all walls, floors and doors enclosing the stair to be fire rated. You also cannot have open plan layouts at ground level which would compromise the stair escape. You also have more onerous fire detection and sounder requirements. All very doable, just be mindful of the impact on the rest of the building. If not properly taken into account it can make mortgage on the property difficult for any future buyers.

i know its cheeky but maybe knock on the door and ask the owners if they converted the loft and which building company they used? This is the most direct route. You could also contact loft specialist companies and request a ballpark quote based on the plans you have posted. The cost will also depend on the specification, the findings from a site inspection and various other factors. You said you have your eye on this property. Have you viewed it or made an offer? If you can show the buyer that you are seriously considering making an offer they may allow you to bring a builder to a second viewing.

Yes I've heard of them, but not used them. They appear quite regularly in my newsletters from https://www.homebuilding.co.uk/

You don’t have to close off the second space unless you want it to count as a formal bedroom on the plans or for Building Control to treat it as a separate habitable room. Building Control mainly cares about safe escape routes, fire protection, and the staircase layout. As long as the main bedroom space is properly enclosed where required for fire safety, the rest of the loft can stay open if it is being used as a landing, storage area, or general open space. Many people with smaller houses keep part of the loft open to avoid awkward stair designs or losing headroom. The key thing is that the enclosed bedroom must meet the rules on fire doors, insulation, and escape windows. The open area usually just has to meet the general structural and insulation requirements. Your builder or architect can confirm the exact layout, but you usually do not have to redesign everything just to close off that second space unless you specifically want two separate rooms.