You did not list a budget
A couple questions:
1st- Would you ever want to transcode on the fly your media for use while away from home, or on other devices like ROKU, ipad, iphone, laptops, android, kindle, windows phone, etc... ? If not- and you are certain you can skimp on the CPU without much penalty, it will serve media over LAN just fine with a Pentium or Celeron for short money. (under $50 CPU)
If you might, then you want an i3 or greater. i3 can't handle multiple streams though, so i5 is safer bet if you want two streams at the same time. I do this all the time. Actually I just remoted into my Flexraid server and checked, wife is watching via PLEX on our ROKU and my parents are watching at their house via Mediabrowser 3 app for their ROKU. At times I join the mix with the ipad, and my brother does at his house with his HTPC or his ROKU. This functionality is really the only reason you need a beefy CPU. You do not need a beefy CPU to just send data over LAN as a server.
2nd, Do you have a space requirement ? How many drives ideally do you want capacity for ? I have 20 in mine, and I need more. I have a Norco 4220 server case. If you read my thread you'll see I had a few cases before that, and I kept upgrading to get where I am. Same story on the CPU, I started with a Celeron, then a Pentium, today I have an i7
My advice is do it right the first time. It's easier.
3rd- Would you ever want support for bigger than 3TB? I have 4TB parity drives in mine just in case I wanted to add a 4TB drive for data, but to date I keep adding 3TB because cost is effective, and also performance is better.