What kind of welding and cutting equipment do you have, and are you comfortable with welding 10 and 16 ga steel?
Have you ever poured a large concrete floor before? You probably want to form it up and have it ready to pour, and have a contractor show up just to pour and finish. The contractor may even be able to save you a few bucks on the concrete.
Can you do your own dirt work? Perhaps rent a box blade and small excavator locally?
Do you have access to a forklift, cherry picker or pole truck during construction?