Think BIG, live LARGE! 
With Xmas coming up, a large batch produced at "bulk quantity" prices could be an attractive stocking stuffer if they could find a way around the Kickstart ROM issue and bundled it with a bunch of the "free" games (Cinemaware's stuff, etc).
My consideration is mainly that any PCB layout bugs, presuming components are OK. If there is any layout mismatch even simple things like footprints. Then correcting 54 boards by hand is no easy task....
Thats why I think no more than 5 should be produced initially to make sure gerbers, pcb manufacture process and components are up to the job. Also things like heat issues, interference, stability etc.. may also screw up things.
Real production starts in the quantity of 1000s of boards. That's the usual minimum quantity to buy directly from manufacturers.
The kickstart-ROM I guess will have to be "found" by users

, the alternative is to make a clone that present the same API, fits the ROM size, is bit-bang compatible at certain addresses, and runs on 68000/68020.