You can start from downloading the latest current version of Ubuntu Mate (17.10), that way you can upgrade the system in a few weeks when 18.04 is ready!.
Stock Ubuntu is using Wayland if you are trying to use 17.10 which you don't want!. It is being dropped in 18.04!. Ubuntu Mate uses "xorg" so you shouldn't have a problem there!.
I advise you to read through the beginners guide which might help you further!:
