Thanks for those links! I will go through them carefully. Doing a startup is what I want to do but before any of that I want to do the due diligence part first and learn things.
You should probably start with the Adafruit guide, it's a good survey and starting point. It doesn't delve deeply into actual prototyping, but it covers the 'due diligence' aspect.
The first half is run of the mill startupy stuff, but the second half has a lot of useful information to consider.
I'm not sure how helpful the blog links will be at this point.
Re: External Casing- If you have access to a 3D printer, I highly recommend trying out a few iterations of the casing yourself before you try to outsource.
Once you have everything working, you can find companies that do limited runs of PCB and casing.