Hello! I suggest the Crucial MX500 Line of SATA drives. They're reliable, and Crucial is a big brand under Micron, an even bigger brand. If you really need, a 2TB SATA drive will be rather expensive because of storage density but will also be more than enough storage. You may also like the Samsung Pro line if money is no object, they've got incredibly fast drives.
I suggest you keep the Hard Drive as an extra storage, and move all your information to the faster SSD by creating images of the partitions of the old hard drives through a tool like Macrium Reflect and imaging them onto the SSD.
Then, you can do what @danj meant by “cleaning" the drive: wiping it clean of files, because they're on your SSD, which we'll be calling the “boot" drive from now on. Since the files are on your boot drive, you can now use the blank HDD, which is no longer the drive you're booting from, as extra storage.
A convoluted way of saying that the SSD is now the master and the HDD is now the extra storage, while previously it was the other way around.
Hopefully everything checks out. These are some of the basics of files, as SATA is a standardized port and so any HDD or SSD equipped with a SATA port will fit in that slot.
If you need help with Macrium Reflect, or formatting to Apple Journaled Extended (Apple's special format for bootable drives) please do respond, I will try my best to answer any general questions because I don't own that product specifically.
Also, that looks like one !&&* of a difficult teardown just to put an SSD in.