How to get a clean install of Windows 10 to activate?
According to Apple's Knowledge Base article, your iMac isn't supported directly in Boot Camp.


https://support.apple.com/kb/HT204990


You should probably get VirtualBox or another VM solution to run the Windows 10 as that would bridge the driver gap better than trying to piece things together yourself. It might be possible, but I don't know where you'd start on it...