I did this awhile ago and didn't spend that much money. Not even close. Of course, I made my own trusses that spanned 24 feet and left the floor gravel. Trusses where shaped like the letter A, which gave me added height. The guy who owns it now parks a cabbed tractor with his batwing attached, and a full sized backhoe in there.
To deal with humidity, put vents down low at one end, near the floor, and on the ceiling at the other end. As the day warms up, air enters the low vent and travels to the high vent, eliminating condensation.
To attach the trusses, I bolted 2x12s to the side of the container. Just drill and bolt them in place. My containers where not perfectly level, so I adjusted the 2x12's as I installed them to be level. I think I was off a couple inches from the front to the back of the container, which you never noticed over 40 feet.
The back and front wall where just posts in the ground, purlins across the posts and T1-11 plywood. I hate that stuff, but it's cheap and effective if you keep it painted.