There's no technical requirement that's forcing you to use a separate license for any of this. You can setup PowerGen to run the build process right on each developers' workstation - just use a different set of PBLs and PB workspaces. Your DEV workspaces point to your local dev PBLs, and the PowerGen build process can point to PBLs on a shared LAN drive. The GLV process pulls the latest changes from the SCC repository right into the shared PBLs and kicks off the build. Just don't do development against the PBLs on a shared LAN drive and everything should go smoothly.
For whatever reason, it was your requirement that this all happen on a separate box - not SAP's or PowerBuilder's... There are license implications to satisfying that requirement. So, it's up to you to determine whether it's a real requirement - or just a "nice to have"...
-Paul-