Typically, for storage 100 lbs. per square foot is used for live load. You can get joist size and spacing from an on-line calculator. You'll need to frame the mezzanine separate from steel structure as it's not designed to carry additional loads (I assume). You could frame a stud wall at the exterior which would distribute the loads and give a wall to plywood and hang tools on or whatever. As far as the posts, one long span will require a very deep beam and the resultant load on one post will be on the order of 6,000 lbs. which is probably more than a slab can carry. If you're dead set on one post, you should have a footing. A lumberyard might be able to size the front beam, or you could get an engineer to size a steel beam and column.

Forklift and scissorlift

The slab is 5" and footer area all across is 3'

Invest in a local structural engineer.

You might want to set a wood post on a concrete pad sitting on top of the slab. That would do two things: distribute the load to the slab, and help keep a forklift or such from dislodging the post base and wreaking havoc. The pad wouldn't need to be very big. If a steel column, just get a large (say 12 x 12 x 3/4) base plate welded to the bottom of the column.