Our clients are a blend of Open Value Agreement with the Windows Desktop Upgrade/Enterprise license moving to Windows E5 to gain Advanced Threat Protection and security licensing for the desktop.
When renewing the Open Value Agreement the perpetual licenses stay with the client and the renewal is SA Only.
Looking long term, an Open Value Agreement pays for itself in the third renewal so years ten, eleven, and twelve all things being equal.
SA is required or O365 E3 and up (I think) for Office to run in a RDS setup.
If more than one Session Host is going to be set up then it's a best practice to put Broker/Gateway/Web on one VM and the Session Hosts on the other.