He is hoping to have this posted by the end of the weekend

Me too, I want to use a Nano.

How is nano different? I am new to this but other than storage size the nano configuration should be no different. Looking at something like the Geekworm NVIDIA Jetson Nano 2.5 inch SATA SSD/HDD Shield Storage Expansion Board to allow for 1 TB of storage.

I think you just answered your own question. Without an on-board M.2 slot, you have to either add in an expansion board or use a USB SSD drive which requires a custom kernel if you want to boot from it. All doable, but extra steps vs what is listed in the README.

EDIT: power is the other difference. The Nano doesn't have a USB-C port for providing power, and the micro-usb port can only deliver a max of 2A which is barely enough to run the Nano itself and will be insufficient once you add an SSD and wireless card. You'll need a 12V to 5V converter with with a barrel connector to power it.

So what's the reason there's a need for an external SSD? I would expect the only disk read/write is from the Dashcam writing footage to the disk.

I'm obviously speculating here, but I'm guessing an SD card (which is the only onboard storage for the Nano) just doesn't have the IO bandwidth to support the amount of reading and writing necessary. They also are really bad at random IO, preferring to read and right in a stream. If it were just camera data, that would probably be fine, but I would assume that there's a lot more going on when you start doing license plate and face detection. The instructions mention a MongoDB installation which is probably going to generate a huge amount of random IO.