I want to convert an outbuilding into an office and am quite experienced/ capable of DIY but am unsure on a few points.

1. As there is no dampproof course or cavity what membrane product do I use between wall and studs?
2. What do you do where the screws penetrate the membrane?
3. What fasteners do you use, stainless steel?
4. Floor is currently bricks and want to lay wooden floor, again what membrane do I use? It's uneven so not sure whether to lay battens with insulation inbetween or pour concrete and stud on top?
5. what thickness cellotex do I need for walls and floor?
6. How do I overlap wall membrane with floor membrane?

would appreciate any advice on these points, also I am gonna run a new Elec supply in from street any ideas on cost . thanks!