Installing & using a unity session in 17.10
This is generally quite simple, just go
sudo apt install unity
When done you must reboot, when back at the login screen click on little cog wheel to dropdown session choices & choose unity.
If auto logging in then after the reboot just log out, at the login screen expose the dropdown & choose unity.
Once chosen unity will then become your default login.
To get the best unity session experience one should use lightdm instead of gdm3, to do that simply
sudo apt install lightdm
At the conf prompt choose lightdm, reboot.
If not intending to use gnome-shell after switching to lightdm feel free to remove gdm3 & gnome-shell, i.e. sudo apt purge gdm3 gnome-shell
There are some minor issues, most will be fixed or worked around for 18.04 but not for 17.10. As one example alt+print will not work to screenshot a window. The current workaround is this, (I've 'fixed' it in 18.04 via compiz & gnome-settings-daemon but hoping for more direct fix) -
press and hold Print Screen
press and hold Alt
release Print Screen (and then Alt)
Also if not getting onscreen notifications make sure notify-osd is installed,
sudo apt install notify-osd