You may have to make a trade-off here. If your are keeping source material on the drive, you primary consideration may be the greatest amount of space for the least amount of money. The opposite of that is if you are using it for your working drive, storing your proxy media, etc., in which case you want the fastest drive possible.
Here's an article that Larry Jordan wrote in 2013 about what kind of drive speed you need:
https://larryjordan.com/articles/hard-drive-speed/
To determine how many drives you need, take the number of video files you are playing
at the same time and multiply that by 20 MB/second. (In other words, an eight camera multicam shoot would be 8 * 20 = 160 MB/second.) [which would likely require multiple drives in a RAID configuration].
I suppose ideally you would have 3 drives:
a. Your boot drive, with the application
b. Your media drive, with the source media. This can be slower than the other drives (as above)
c. A work drive, probably an SSD, which has your proxy and other files which benefit from the faster speed
Is this going to be an internal or external drive? How is it going to be connected (SATA, USB (2,3)?)