Have you considered used server parts? You can get a dual E5-2600 V2 board with 64GB of ram with processors for under $500 delivered. Add a Rosewill 15 drive case (not hot swap but hey - it's your 2nd back up) for $120, whatever power supply you want and a $50 HBA and be up and running for under $750. Jjust a thought.
I am not against using older hardware but the noise/power do concern me. Someone on the board suggested this: http://www.ebay.com/itm/Supermicro-...508377?hash=item5683dd5699:g:msoAAOSw5VFWFvJa
With a M105 HBA (might have the model wrong) HBA for my 6tb WD red sata drives.
I am trying to scale down some of the 24/7 towers I have running for various tasks with VM's to minimize the electric bill & noise/heat - so I am not sure if this is a great idea.
I am also looking for a reliable backup - so I dont know if something used is more or less reliable - new & old things do break.