All SU major point releases (ie. SU2016 / SU2017 / SU2018 / SU2019 / SU2020) run independent from each other and can be installed on the same machine without the one influencing the other.
I doubt that any versions of SU, apart from the current release, is available for purchase. You could communicate with SU though, but the cost won't be less than the current version if they were to meet you halfway.
Don't go subscription, perpetual licence only - IMHO anyway.