For what you have described, a Xeon is overkill. A modern day Pentium processor will provide enough horsepower for file sharing and running Plex, even transcoding one video stream. Pentiums also run cool, which is a big plus in my book. If you want a little more headroom, a Core i3 will give you plenty of power for running Plex and transcoding two video streams. I'm running a Core i3 3240 and have not yet pushed the processor to its limits.
Don't worry about using a previous generation processor. You will not push the processor to its limit, so save a little money on the processor and put it into RAM memory. If you plan on supporting 8-10TB of storage, a minimum 16GB of memory is a must.