Is there a way to disable the GUI in the desktop version of Ubuntu 18.04. so save on power consumption during idle states?
Since I won't have a monitor hooked up to the machine anyway, I'd rather manage Ubuntu through an SSH connection or Webmin.
Yes, your desktop is started from a systemd webservice. Disable it and you can still manually start it.
systemctl list-units --type service --all will list all services. For me it lists "gdm.service".
This should show the status of your session:
systemctl status gdm.service
This disables it:
systemctl disable gdm.service
And this enables it again:
systemctl enable gdm.service
Replace it with kde if you use kUbutu. Or by any of the other desktop you use.
You might need to kill the current active desktop too.
If you intend to use this system as a server you can also delete the desktop itself with
sudo apt purge ubuntu-desktop. Be careful: do this while you have a ssh connection so you can still issue commands; locking yourself out of the system might be fatal.
For Ubuntu 18.04, the following will work without additional modifications.
systemctl set-default multi-user.target