Hello, Reduced activity on the forum was likely a result of, first, Chinese Spring Festival, second, the Coronavirus quarantine. Only recently have some of the Khadas Team been able to return to their offices.
A TF card or Transflash card is a micro SD card. The VIM3 has a micro SD card slot.
Versions of Ubuntu, Armbian and others, both from the Khadas Team and other developers, are available in both desktop and server(headless) varieties.
For Khadas Linux firmwares, see here from the Khadas Docs.
See @balbes150 posts for some great Linux options. See @Spikerguy posts for a nice Manjaro option.
Case, Power supply and other accessories can be found in the Khadas Shop.
I use a CH340G and a Prolific 2303 TTL to USB adapters from PC to the VIMs.
For the rest of your inquiry, I will leave that to those more knowledgeable on Linux than I.
Additionally, you may find Krescue of interest.