I think the money would be better spent on more harddisc space. As long as you are going USB3 or direct SATA, a decent computer should be able to get through the drives in not much time. I am through my roughly 20TB on each box in a minute or less, even using the reference java client. I have one java miner running for each drive, so they run in parrallel and get eaten quite fast.
A good GPU second hand will cost less than a 4TB drive - I've got some R9-280X's going for $150 shipped if anyone is after one.
Round times vary, but on the last block one of my miners did 36TB in 21 seconds, another did 55TB in 45 seconds. All the parallelism is in the client, so one instance can mine 10 drives.
H.
By the way your price of used gpu is veey high
New R280x price almost 140 usd
take alook
http://h5.m.taobao.com/awp/core/detail.htm?id=524333051092&sid=caceddf427fdad5968261e3588879e95&rn=b8226de67e4df219fa5740496840840e&abtest=22