They still build King Airs, and I think those are built in much the same way as they always have been.
The advantage of the 1000 is that it's already certified, with modern avionics and everything, and some of the tooling (although maybe less than would be hoped for) exists, along with a bunch of PMA'd parts, etc. So it would seem to be MUCH cheaper to get started than with a clean sheet design.
The other advantage of the 1000 is that nothing really does what it does. It's a great competitor to the PC-12-- a little bit smaller cabin, but faster and more efficient. And twin engine.
I'm sure what they need, though, is a 25 or 50 unit order to get started. Either some sort of fractional or special mission program.