I have a very similar setup using a HP Proliant micro server. http://tinyurl.com/c3g5prl
I installed Freenas on it and run ZFS (Similar to Raid 5)
It has never gone down or died on me in ~ 2 years and was pretty easy to setup.
Heres a link with loads of into that I learnt a lot from. http://tinyurl.com/6d3lz57
Other options would be to buy a preconfigured nas box from Dlink, Qnap, Synology etc, but they are more expensive but easier to setup.