Hi Ross,
I haven't heard about any legal requirement or Solman is mandatory.
Solman is used for Project Documentation, System Monitoring, New deployment, License management and similar other configs and YES it is mandatory if you have small to huge landscape for maintaining your systems properly (monitoring tools etc.).
But in your case, only one ECC system is getting installed and you won't be applying SPs or Add-Ons so I think you are good to go. I think you won't be needing help from SAP also (SAP Router config, Message creation for issues.).
If you still want confirmation then you can directly call at SAP Helpdesk number and they can update/confirm you.
Thanks,
MPS
