Go for a cheap OEM key. You can get them for usually 5 bucks or so, also download the Window's ISO from Microsoft itself. Don't use any torrents for getting the ISO itself, you can never be sure with those ISOs.
Use this website to download:
https://www.microsoft.com/en-gb/software-download/windows10ISOAlso, it will make you download their shitty "download tool" however you can get the link to direct ISO file by doing the following.
1) Visit the URL I mentioned above.
2) Open your console F12 usually works on Chrome.
3) Enable "Device Toolbar". See image 1.
4) Change your Device to an iPhone or iPad. See image 2.
5) Refresh the page. After the refresh, you will see an option to select your windows ISO file. Just choose the latest version and choose the appropriate version for your CPU, x64 bit or x86 bit. Whatever works. You should get the direct ISO link this time. Use this ISO when you activate your windows.
Image 1:image 2: