Holy Tobasco! This is the most exciting Amiga news in years.
Count me in. Fully assembled, 12 x 12 would be better (so long as someone comes up with a custom case that we can buy as an extra). Otherwise mini ITX might be better.
I think it's fair to say that you'll get an initial run of 100 of these things, no problem.
As a suggestion, don't under-price them. Figure out how long it takes you to assemble and do basic tests on a unit, and then price accordingly. You'll need to be motivated to keep going!