Looking for a cheap storage server (recommendations)

Hello HomeLab!
I'm looking for a used/refurbished storage server that won't break the bank.
(bank being ~$650)
It's purpose will be to store media and serve it to any of my home devices, store backups of our machines, etc. I'd love to have a server (no drives, I'll buy those as I go and as funding rebounds) that has a ~30TB capacity. I've read the sticky about choosing processors, ram, RAID cards/SW, etc. I've seen the R710's, and they look great! Thinking about buying two or three of those later to practice on, but they don't have the storage capacity I'm looking for.
If you guys could give me some models to research and get me looking in the right direction, that'd be great!
Thanks^ahead^of^time.

Edit: I'd like to run this machine in Raid 6, with SATA drives since they seem to be cheaper at high capacity.