Hi Mike,
Sarking is definitely a good idea - especially if you get a lot of dew at night.
I was going to say get a couple of batts for insulation and to keep it up there, use some chicken wire, but I think Ballaratdragons idea of styrofoam will look neater - could probably use liquid nails to stick them on the ceiling.
As for the floor, you could lay a slab quite easily, alternative buy some pre-formed slabs (900mm x 900mm). By the time you get a number of these, making the concrete yourself may prove cheaper.
Will there be any power to the shed?
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