I need to buy or build a Raid 5 NAS system that is somewhat reliable. 70% of its use will be for storage (such as for back ups), the remaining 30% will be for accessing files from the Intranet. 

Looking at an entry level NAS system (4 or 5 bays) I found it to go from $200 to about $800, an amount of money that I cannot afford.

I have a working PC sitting in the storage room unused with these specs: an AMD Quadcore, 8Gb Ram, a standard 18" display, a video card (unsure what kind) and of course a MB (also unsure what kind). Eventually i can add a 120Gb SSD to run the OS.  

Somebody from AskUbuntu suggested me to look into FreeNas and I was wondering if it is difficoult for a newbie to create a NAS system with FreeNas on this separated PC. I plan to add all my spare HDD (5 x 250GB).