Plex loves cores and clock speed. Even though the new transcoder is more efficient than the last iteration, it still likes clock speed and cores. I have a Plex server with 6 core 1035T . I can stream a 1080P 10Mbps Bitrate movie at the same time to these devices - All transcoding -
Roku 2 XS -1080p - 10mbps
Droid DNA - 1080P - 8mbps
iPad 2 - 720P - 4mbps
iPod Touch 4th gen - 720P - 4mbps
Anymore devices and it chokes
If I watch a 4-5mbps bitrate movie, the server can handle up to 8-10 transcodes.
So the real question is what bitrate do you plan on ripping your Blu-rays at? Audio and Video. And you don't have to transcode everything. You could always rip them in a MP4 H.264 and they would direct play on the Roku and PC's, not sure about the i Devices.
Keep in mind Roku states there top tier model Roku XS, can handle bitrates up to 8mbps. I have done up to 15mbps bitrate movies on the Roku, but the server has to be strong enough to transcode within the 10 second HLS segments it's given. My old X3 struggled, but would do it.
My advice would be to rip a blue-ray at an acceptable bitrate and try it before you buy anything. Also, how many transcodes do you think you will be running at a time?