If you're buying a new PC:
Go ahead and get it with Windows 10. Windows 8 and 8.1 machines are about to become ghosts.
If you're buying a Windows 8 or 8.1 PC and looking for bargains:
Probably too much work to buy an 8 or 8.1 machine and upgrade to Windows 10. There might be driver and compatibility issues. That said, if you want to be frugal and can live with the probable pain, go for it.
If you're buying a PC with Windows 7:
If you're buying a Windows 7 machine now, it's because you want Windows 7.
If you're building a new PC:
You'll want Windows 10, especially if you want to do cool things with games and VR.
If you're running Windows RT:
Toss it in the trash and buy yourself a useful machine. There might be a chance you can upgrade to Windows 10, but you'll probably run into storage and other hassles.
If you're running real Windows 8 or 8.1 on a traditional PC:
Upgrade right away. Windows 8 and 8.1 are about become forgotten to history.
If you're running Windows 8 or 8.1 on a tablet:
Probably best to stick with 8.1. If you're running Windows 8, move to 8.1. Windows 10 might work, but it might not be worth the risk.
If you're running Windows 7:
Unless you absolutely love one of the Windows 10 new features (most notably Cortana or the gaming features), you should stick with Windows 7. I'm not upgrading my Windows 7 machines.
If you're running Vista:
Find thee a dumpster.
If you're running XP:
Look, you know who you are and why you're still running XP. You should stop, but you're not going to, are you?