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I re-read your initial post and i guess you're planning on having the slab as the floor?
In that case, you're probably better off re-pouring if you have doubts about the existing slab.
My building is primarily for working in, although bikes are stored here too. I wanted to raise it (actual floor is about 20cm off the ground) so i could insulate the floor and heat the room without worrying about condensation, but you presumably wouldn't have those issues.
I've just built a garden office type building (it's timber construction though, not brick). I opted for setting it on concrete piers rather than a slab - plenty of airflow underneath it and no need to worry about drainage as the joists are all at least 4 inches of the ground. I did have to remove an existing concrete base - nearly re-used until i gave it a whack with an iron bar and found it was only an inch thick :-/