joeschmuck's advice is well taken: think about what you want to do...
While this sounds obvious, it's amazing how many people start building a system before thinking through the important issues. The first decision: what functionality are you looking for? File serving? With or without encryption? Transcoding media files? File serving requires far less CPU power than transcoding media.
How much disk capacity and how many physical disks do you expect to use? Obviously, this will depend upon your RAID strategy. It pays to have a plan before you start buying hardware - if for no other reason than to make certain your motherboard has enough SATA connections and your case has enough room for the disks. DON'T USE OLD JUNK DRIVES. This creates headaches down the road.
Related to this is "How large a system do you want". By this, I mean physical size (dimensions.) Physical size doesn't get talked about a lot on these forums, but I think it should be considered. There are lots of folks out there who build a NAS using a mini ITX motherboard in a small good looking case, and then complain they can't add enough memory or internal hard disk.
For many years, I ran FreeNAS on an ancient AMD 939 class CPU as a file server and it worked great. I started serving media files using mini-DLNA and it still worked well enough. Then, I decided to add the ability to transcode media files for mobile devices... no go on the old system. Time to upgrade. One comment about my old system: I updated the disks from time to time, and ran that system for 7 years without incident. Do not rely on old disks as they have a tendency to fail at the most inopportune time.
joeschmuck's setup is a great example of an AMD based system. Personally, I wanted to try an Intel based system as the CPU's are more power efficient. Now, I'm running a Supermicro X9SCL-F motherboard, Core i3-3240 CPU, 8GB ECC ram, 2-2TB disks in a mirrored array. Due to budget constraints, I started with 8GB ram and 2-2TB disks which is enough for the capacity I currently require. I will grow this as budget allows. The Core i3 is a bit overkill for my purpose, but I got a good deal on it. A Pentium for a media server would have been just fine (a Celeron would be sufficient for a file server and they run very cool.)
My opinion about hardware: Buy good hardware. If budget is an issue, then start small and have a plan to grow your system as budget allows. Do not compromise the quality of the components you purchase. I see no problem using previous generation components, which can often be found at discount prices. Remember, a NAS does not need to have the fastest CPU or memory - but it needs to be reliable.
And yes, IPMI is cool.