Yes you absolutely can run a Plex server off this. That is exactly why I got mine. It depends on your level of comfort with linux but it can be done. First you will need to download a version of ubuntu that is specifically ported to work with the Pi 2 at https://wiki.ubuntu.com/ARM/RaspberryPi
and follow the directions on getting it installed to the pi SD card.
once that is installed and you are able to boot into it then watch this video on how to set it up for plex media server. I suggest you keep a laptop nearby so you can follow the video as you are setting up the device. Video to watch is: https://www.youtube.com/watch?v=9jdbwlg ... 91&list=WL
Your external harddrive needs to have it's own power supply, the pi doesn't have enough amperage available to run itself and a hard drive. Something like this one will be perfect: http://www.amazon.com/Mediasonic-ProBox ... +enclosure
Good luck hope this helps you out
Obviously you have not tried your own 'solution', so why offer it.
What you are suggesting will never work as there is no Plex Media Server package for the Raspberry Pi available, there are packages for ARM processors, but these will not install the way you are suggesting and using the Ubuntu package is even a more ludicrous suggestion.
The RPi2 will run PMS, and will serve a limited number of Plex clients (i.e. Roku, Android, iOS, Samsung Smart devices, Rasplex, XBOne, XBox 360, Playstation, Plex HT, etc) but only by Direct Playing supported media, transcoding is out of the question.
I have PMS running on a RPi2, with an SSD external drive (contains the media), serving iPad's and iPhone's in the car and a second RPi2 running Rasplex when we're away from home.
Instructions can be found here https://forums.plex.tv/index.php/topic/ ... /?p=846534
Hardware: 1xRPi B 256Mb running Musicbox, 2 x RPi2's running Rasplex, RPi B+ running mail and web services, RPi2 running Plex Media Server