There are turnkey manufacturers who will build all this for you, but the starting place is a manufacturable design. If you started with off the shelf modules, then one of the first steps would be to get there. So, schematic development and PCB layout to come up with a single PCB that incorporates everything, into an off the shelf plastic housing would be the first step. Then you've got something to build 5 or 10 of, and see if people want to buy them. Bunnie Huang's blog has some good material on this.