Do an upgrade first(from inside Windows 8 run setup.exe of the mounted or burned ISO). Once this is done, your system will activate. After that, use any media you want for a clean install or perform a clean install from inside Windows 10. Use any ISO you have downloaded as long as it's valid and the same as your upgraded Windows 10.
Yeah sorry, I was just over a thread where a guy did everything in his power not to to the upgrade process properly, despite being told a couple times and asking the same question over and over again. Basically the tool will create the ISO (or download it and start the upgrade process right there and then). If you have Windows 8.1 Core you want to upgrade to Windows 10 Home, and if you're running Windows 8.1 Pro then you want Windows 10 Pro. If you already have the ISO then go ahead and use that one instead of using the tool. Sounds like you know what you're doing, see you on the other side of Windows 10.
Use Rufus to create an Install Disk. It will install in about a 1/4 of the time than from a DVD. Good Luck Web