What's the point of planning the whole project to conclusion before doing a prototype demonstration?
Ideally, it would be a successful project - that's what planning is for.
The way you were talking, though, you made no mention of outsourcing everything; you seem to have no experience with a large scale project of this sort (neither do I, of course) - it just seemed like you were doing "everything"...
So - if you are only the prototype developer, and everything else will be outsourced (by the client?) - what do you care what happens after the prototype is developed?
Now that I look at your initial post; maybe the format of the sentence structure is throwing me - are you instead actually asking how to integrate a pro mini, a custom PC board, a GSM module, and I2C expanders into a case or something?
I thought you were the prototype developer (who should have experience on this)?
:-?