A wooden shed should have less condensation then a metal one. The shed itself isn't your real defense, it is just weather proofing and for keeping out of sight.
For anchors you could get one of these from B&Q and screw it into the ground under the slabs with just the rings showing. See also
this thread.
Place any anchor between the bikes and a solid wall of the shed, or the side of the shed that is against a solid wall, so that it is harder to get at.
If you go for a bolt down anchor go for this type so the chain cannot be slipped out with the removal of one bolt.
It is better then the open type of anchor like this.
If you are bolting down onto slabs try to use pavement slabs and not garden slabs. The pavement ones are thicker, bigger ad tougher then the garden ones that are as tough as a cream cracker. Otherwise pour some concrete to secure the anchor under the wooden floor.