Garden Office - Timber Frame Construction Techniques

I am about to build a small garden office (8ft*8ft) and was looking for some advice re building methods. Any help/comments greatly appreciated.
Flooring
I am going to build a deck frame so I can raise the building off the ground. Base will be 18mm OSB on 4*2 treated joists. Below the OSB some kind of insulation. After that I'll lay a wooden/laminate floor, possibly underfloor electric heating.
-- Should I add a layer of Damp Proof Membrane between the joists and the OSB board?
-- Should I add building paper on the side nearest the earth (below the insulation)?
Walls
Timber Frame construction. T&G Cladding, building paper, 70mm*43mm Joists, insulation, T&G inside to finish.
-- Is there any advantage in 'skinning' with the walls like of a 10mm ply/osb under the cladding?
-- Do I need a vapour layer on the inside? I've been looking at the likes of the "Breather Foil for Timber Frame" or the Airtec Double Inuslation.
-- Anyone use this stuff before - http://www.superfoil.co.uk/SF19.html?
Roofing
I'm going to use Felt shingles on 12mm OSB.
-- Do I need to add any other damproofing/ membrane?
Windows/Doors
I picked up a couple of 3*4 maghonany windows second hand. I'm looking to pickup a couple of french doors off ebay
-- Is there any huge advantage in buying a door frame (48") rather than just building a frame using 4*2's?

Thanks in advance for any help. Andy