In regards to upgrade path, you can refer to the R1 release notes of each major release which will state what is the recommend upgrade path. From 6.5, you will want to upgrade to 7.1 > 8.1.
Since NC is part of the server software, this will be automatically upgrade when your end user connect to the PCS device after the upgrade. Installer service is only needed if end users do not have admin rights to install PCS components. If this is the case, you will need to push Installer Service before before the upgrade process.