I have been a metal building contractor for over twenty years so here are a few questions. Are you dealing with a building department? Do you already have permit plans prepared to put out to quote? Does having water in the building affect the energy design of the foundation and building with the building department? If you have plans do they take into account any collateral load required?
Even if you decide you will be your own contractor I would strongly suggest that you hire a metal building contractor to provide and erect the building. I am the guy you call after a week of frustration to come out and finish the erection. I show up with a crane, telehandler, a couple of sizzor lifts and six guys that know what they are doing and start hanging iron very quickly. By the way 8% is a good deal, he is almost working for free.