Hudson
I'm making some assumptions, so forgive anything you have already planned for or have already thought through.
I'm sure you have thought about it---but if you plan on hanging any heavy bags, you need to make sure the beams will take the weight/stress over time.
I only mention it b/c you mention pre-fab steel, and some of those that I have seen use ridged interior sections, not well supported beams.
Depends of course what exactly you are looking at.
In terms of toilet, how close is the dojo going to be to the house and how ok are you with people tramping from the dojo and back? Would it need its own septic system or can you easily connect it to the houses system?
Depending on who you might want to lease the building to the "matted section" might have to be able to rolled up for those that need hardwood.
Concrete block is common in many places.
I'd go with whatever is cheap and sturdy and goes up quickly. But I'm a minimalist at heart and a stark, industrial "look" is ok with me.
If you get a chance I'd love to see some pictures of what it eventually looks like.
Chris Thomas
"While people are entitled to their illusions, they are not entitled to a limitless enjoyment of them and they are not entitled to impose them upon others."
"Team Cynicism" MVP 2005-2006
Currently on "Injured/Reserve" list due to a scathing Sarcasm pile-up.