I've had only limited experience with Windows 8, on a pretty weak computer, so I can't really give a decent opinion on that one. I find Windows 10 annoying but reasonably easy to use, once I figure out how to force it to cooperate. TS3 doesn't even recognize 10—it thinks I'm running some version of 8—but the game itself runs fine.
Regardless, my computer wouldn't support anything other than 10 anyway, so I didn't bother to research the choices. I think that your choice of OS depends on your version of Bootcamp Assistant, so you might be stuck with 8.1, but you'll need to check your version number to be sure. (Even running Mojave, your computer might not support the latest version of the assistant.) You could also call Apple and ask.
The reason I'm not completely sure how this works is that with my mac, I just downloaded a Windows 10 ISO file from Microsoft, then pointed Bootcamp Assistant at it and followed the instructions.
https://www.microsoft.com/en-us/software-download/windows10ISO
I believe the same principle applies for a USB. You could buy one, or you could create one using a computer with an existing Windows install. If you use the above link on a computer running Windows, you get the option to download the media creation tool, which will let you burn a disc image (ISO file) for Windows 10 onto a USB or DVD. The same principle may apply to a Windows 8 download, but I don't have an install on which I can test. Here's the link where you'd download it:
https://www.microsoft.com/en-us/software-download/windows8ISO
And keep in mind that the process of burning the disc image to the USB will erase any other data on the USB.
It might be better to call Apple and ask the proper procedure for your specific model, just to be sure. But for Windows itself, if everything goes wrong, you can always wipe the partition, and your computer will be back to normal.
On Amazon, you can buy a standalone code, a DVD, or a USB of Windows 10. The entry should indicate what you're receiving; if it doesn't, then it's probably just a code that gets emailed to you once you pay. (I haven't looked up what's available for 8.1.) You can also buy directly from Microsoft, but that's often a bit more expensive. For example, Microsoft is currently charging $140 for an activation code for 10, whereas I found my mom a USB stick with version 1703 (current is 1809) for $109, no tax. I was Bootcamping my parents' new iMac though, so I could download and run the ISO file directly and didn't need anything other than the code that came with it. I'm not sure you can even get a USB with any newer version of Windows, and Amazon doesn't list which build you're getting.
Anyway, some of this can wait until you've decided which version of Windows you want, and whether you can use your dad's computer or need another source.
