Something else to think about is future expandability. I'm assuming you'll start with an HDD or two, but what about after that? You'll either need to purge out your library or have a bunch of external drives connected. The Optiplex is a great thin client but it only has 1 or 2 SATA ports and one will likely always be used for your OS drive (SSD) which leaves only one internal slot for an HDD.

Also just some random advice, but if you want cheap drives, keep an eye out for WD Element or WD EasyStore external drives to go on sale. These are extremely easy to remove from their case and be used as an internal drive. Something else to check out would be serverpartdeals.com if you don't mind trying some used or refurbished enterprise drives. I bought my last 14TB drive from them for something like $100 and haven't had any issues. This is generally worth the risk since you can just download your media over again if the drive happens to crap out on you.