double-
It sounds like this could turn out well for you, the guys upstairs, and your friends and future costomers. You've also received a bunch of good advice and much to consider. One thing you didn't specify is whether the business owners rent their space or own it. If you're paying rent there should be a lease agrement which should protect you as well as the lessor including having adequate hazard and loss insurance. Also, if the lower level space you intend to occupy is a basement or even street level, I'd check the flood insurance rate map (FIRM) to see if you'll be wet or dry in the event of a flood...
Best of luck to you!
jmc
