The problem is that if you want some reliable NAS you need to spend at lesat $30k
These NAS "servers" you point in the ebay link are a SPF (Single Point of Failure) equipment.
This is something you could call a NAS:
http://shop.sun.com/is-bin/INTERSHOP..._US-SunCatalog
http://shop.sun.com/is-bin/INTERSHOP..._US-SunCatalog
Now that you know the price for a real NAS... you should know that in most cases there are failover solutions that are much cheaper and give you very similar grade of performance and in most cases custom build storage clusters setup allows very fast recovery on a full server or multiple HDs crash than a NAS with a couple of HDs crashed at the same time.
If you think a RAID 5 will save your ass, then you should see how easy is it to crash 4 HDs at the same time when things go wrong.
If you are interested in some custom solution, then make me know. My hosting company specializes in clsuters and redundant solutions.