If I were going to do something like this, I would probably approach it in the following manner:
-
Draw a schematic for the prototype. Include all parts; design using the ATMega in "bare bones" configuration; include programming header for FTDI cable.
-
Build the prototype from the schematic - use stripboard or something; ATMega328 bare-bones with bootloader, put on programming headers and use FTDI cable to program it (or include the mini - though a mini seems like overkill, but for a prototype, who cares). You would probably need breakout boards for the I2C expanders and GSM stuff to get it to fit. Write the code, get the thing working, perhaps stuff it in a box (I am not sure where you are at on this point).
-
Once the stripboard prototype is verified and working (and, if you went with the mini - you might want to try the bare bones version next - prototype 2), design the PCB. You seem to not have experience with this - so it might have to be something you would outsource; give the designer the revised schematic and such (they might even need a prototype to verify things).
-
Once you get the PCB design made, have one or two etched and drilled by a PCB maker (several available worldwide - actually, I ran across one somewhere in SA just yesterday - seemed like a small outfit). Bring it home, stuff it with parts to verify the design is working; if not, check for problems, check the PCB design, check the prototype, etc - repeat steps 1-4 until it works.
-
At this point, you should have a working PCB layout, and a parts list. Find a combo board maker/stuffer - or have the boards made at one place, and stuffed elsewhere. Do a small run first, of say 10 boards. If those come back working and test out fine, then it just a matter of ramping up the production with the manufacturers you are working with.
-
Once you have a working and final sized board (although you may want to work with the PCB layout designer on this first), find a manufacturer for the cases to fit; sometimes it is easier to pick a case or a manufacturer, and get the sizes needed for your board designer, then design the board around the case dimensions. Otherwise, a custom made case would be needed (necessitating costly tooling and such - not a cheap thing to have done).
Once again, though - I have no experience in any of this, so take it with a grain of salt and do more research; the above is just what I would do based on my minimal research into how you go from a design to a prototype to a product - there are still many gaps in there, but I think the major basics are covered, but once again I have never actually done this. But the resources are there.
Good luck.
