So, if I take an old hard drive that has windows 7 on it and I get a new ssd and download windows 8.1 on it, when they are both in the computer will it cause any complications? I want to be able to use windows 8.1 as my os and then wipe the hard drive completely with a data destroying program. So that I wI'll have a fresh hdd for random stuff.
You will need an external hard drive to back up your data (you cannot access the HDD with Win7 from the SSD with Win8.1) As for complications, the BIOS decides which drive will do the booting, if both have an OS the non-booted won't be accessible but not erased. This is the easiest solution and, of course, you have to re-install all the games (steam allows to copy the files so no re-download is needed)
This comes from my experience dual-booting Gnome and Win8.1 (both boot from the SSD so I can access the HDD from either but not the other OS-written-part in the SSD).
P.S. I have never used the upgrade feature in Windows, nor I trust it.