One of the goals for my build was to try and stay cheap, but still milk out some good performance. I decided to go with the odroid XU4 board. The enclosure is a Mediasonic USB3 4-bay enclosure. I designed and 3d printed some parts that allowed me to mount the XU4 board on top of the Mediasonic enclosure.
my case models for printing - http://www.thingiverse.com/thing:1125745
handy starting guide for configuring OMV on this hardware - http://obihoernchen.net/1235/odroid-xu4-with-openmediavault/
$175 cost before hard drives
low power usage (not as low as other single board computers, but still pretty good)
~90MBs sustained speeds
temperature sensors and fans for HDDs and CPU
SMART status is reported for each HDD and openmediavault can access it.
8 core CPU + 2GB ram is more than enough for plex to transcode audio and video on multiple videos at once
4 * 3.5" hard drives
sata3 to usb3.0 bridge = 5gbit max bandwidth for HDDs