I'll leave it to those with experience to the specific boxes you listed for the first question.
Unraid loads from the USB, but runs in RAM. It doesn't technically get "installed" anywhere. After the boot process is done, only config and state changes are normally written to the flash drive, everything is running from the RAMDisk created at boot.
USB drives can be a little finicky, so searching the forums for specific models known to work would be advised. I'd recommend smaller if you can find it, as there really isn't much it's used for. My main do everything 13 drive multiple VM multiple docker pro box is only using 1.7GB on the flash drive. 4GB is plenty, but may be hard to find a quality new drive that small.
SSD cache drives are very much recommended, especially for plex metadata, it speeds up the browsing and processing appreciably.
I leave all my active machines running and spun down, my backup machines typically only run when needed. The small bit of power that is used by a spun down box is a welcome addition to the climate during winter, and not too onerous in summer. Computers, after all, are small space heaters with a healthy dose of air cleaner mixed in. Dust removal keeps the heater portion of the computer functioning properly. :-)
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