ExtremeTech reviewed the TeraStation Home Server. They ask the question whether you should buy a pre-built system, or D.I.Y. it.
If you build it yourself there several components that you will need to get started, such as: motherboard, case (with high wattage a power supply), drives, RAID controller (if its not already built into the motherboard), gigabit Ethernet card, and OS/NAS software (there are some prebuilt Linux distro that can handle this). After you have all the parts you have to worry about setting it up and configuring it.
Either way its your choice. If you you're in the market for this type of storage this might be a good place to start. Read the article for more information or check out these pictures.