1. how much prep needs to be done? do you need to add fill, or remove
it? that is back-breaking labor but you could do it yourself. if you
wanted to use rebar (optional) you could install that yourself. but
you'd after find the mason first and see what he wants.

2. better call a few masons and see if they can give you a ballpark.
around here, the marterials for a 4" slab would go for about a grand to
1500.

3. you'll want a vapor barrier--six mil poly at a minimum, taped at
the joints and caulked with a caulk like tremco around the perimeter.
I would also put down a layer of extruded polystyrene insulation if you
are in a northern climate. . 2" is nice, but anything would help. Make
sure you get foam rated for underslab, dow blue or ceritfoam green or
something similar.