I have an existing ubuntu 17.10 desktop install.
I've heard many great things about this MATE desktop environment. For most of what I know it should work as a regular shell. How should I install the Ubuntu MATE session?
sudo apt install ubuntu-mate-desktop
which will add the Ubuntu-MATE desktop to your system (including all apps that come with the Ubuntu-MATE had you installed it).
At login time, you can select if you want GNOME, MATE for the current session. If you want to switch, logout and you can switch the other DEsktop.