Welcome aboard sir! Putting on me architects hat for a moment, you should leave the floor insulation as is in the attic. Otherwise you'll end up heating the attic unnecessarily when you're not there. From a cost perspective you could insulate the attic for very little money. If it's a relatively new roof, sheets of 8 x 4 75mm rigid foil faced insulation is about €32 a sheet, and cut with a handsaw can be wedged into the gaps in the rafters. Leave a small cavity between the roof felt and the insulation, and you'll have a space that is quickly heated, but will be balanced in the winter and cool in the summer. If you want to sheet it out later, bevel edged plasterboard which can be taped and jointed will save a few bucks too. No need to plaster when finished, just fill the joints with jointing compound, and a bit of sanding when dry, couple of coats of paint, job done!
Avoid the B&Q's if you can. You'll get better value from a builder's yard.
You'll be able to get a roof access ladder from donedeal or buy and sell. You won't have much change from € 500 for a new one though.
Right, back to the modelling! If I had the options you have i would design the attic layout first, and use the shelf layout to create a part of the scene, so you could do a freight terminal, station building or a large scenic element like a bridge. Then connect it into the main layout when complete. It might work over the access hatch to the attic as the roof ladder is nearly always put beside a gable wall to suit the landing below.
I hope that's of some help and keep up the tidy modelling. Treat it like gardening - little and often!
Riche.