I have this:
Amazon.com: Synology America DiskStation 8-Bay Network Attached Storage (DS1815+): Computers & Accessories
Running 8 3 TB Western Digital Red drives. Don't go cheap on the drives, they will burn out. Get server level. This particular unit allows for the expansion units as well, I believe you can string together up to 18 drives.
This particular brand also has a 12 Bay model. In your scenario, I would probably get that. My drives are getting pretty full.