Gareth,
There is an app u can install from Steinberg that allows u to install Cubase on newer macs:
I used this to install Cubase 6 on Yosemite.
Recently I upgraded Yosemite to High Sierra and for some unknown reason Cubase is using 4 times more processing power in the Mac Activity Monitor. This turns the fans on high. It also causes crashes. When the mac is overheated it crashes.
If you care about running Cubase 6 and your mac is currently on Yosemite, it is my experience and advice that you should stay with Yosemite. I was finding that some apps do not work on Yosemite so I upgraded - however it messed with my Cubase. Cubase 6.0 still works on High Sierra - it just uses more processing power and crashes where previously it was rock solid.
Congrats on getting back into the studio
John