I'm going on a tangent on my job to make a proof of concept that would later be presented and maybe funded so I have 0 dollars to put on this at this point.
My guesstimate is that once everything is said and done we would need about 20 units. But It's a wild guess. It could be way more.
If you wanna discuss this further hit me at me[at]tucaz[dot]net.
I made a handful of units for one company last summer, so the dev cost is covered already. I'd be willing to build a few more. How many do you need?