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.
Sd-Card is suitable only for running something over it but not for continuous read write. So either make full use of the onboard eMMC or boot from emmc and store data on any external drive connected over usb3 else use sd-card of class10 or above
This should depend on the type of application you want to run on the SBC. If you need the most powerful SBC so far available then you need to go with VIM3 as we have almost all known SBC we work with but only VIM3 can compile the heaviest software possible in a very sensible time as compared to other SBC’s with almost the same specs.
It is not too hard to enable watchdog on most of the distros.
You can use both. Using ssh with IP or connect RS232 using this tutorial.
SSH over Lan IP is much better than over SerialPort(RS232).
You can do this easily with any external drive connected to it using usb3.