As an avid Nintendo collector and a person with a recent interest in electronics like Arduino, it made sense to contribute to building a custom Classic NES since they were all sold out this past holiday season.
I want to note a couple written tutorials that helped make this happen:
https://supernintendopi.wordpress.com/2013/01/23/an-a-to-z-beginners-guide-to-installing-retropie-on-a-raspberry-pi/: This tutorial is good to understand how to get the software using the Win32DiskImager tool. However, stop following along with this tutorial when it starts to walk through the EmulationStation options.
https://github.com/retropie/retropie-setup/wiki/First-Installation: This particular tutorial is great for everything else. In fact, it does not cover the EmulationStation options at all. And it turns out that those options are never needed.
https://github.com/RetroPie/RetroPie-Setup/wiki/Running-ROMs-from-a-USB-drive: Details on mounting from USB instead of SD Card.
And here are some links on the materials:
(Pi 3, Power, Sink) https://www.amazon.com/gp/product/B01C6FFNY4
Finally, there are a couple videos that walkthrough the process and teach how to separate data from installation.