Running Plex maybe a bit hectic for this poor bugger, but a lot of people have done it successfully though.
My micro is purely a NAS and I 'stream' if you want to call it that to all my other devices. Got Windows 2008 server install and it even runs game servers without a problem. So the CPU is not that bad.
For IR receiver, well it's nice to have a keyboard type of remote like the RII. They are a bit pricey, but entirely up to you how far you want to setup the system.
http://www.uprice.co.za/p/Rii-Mini-i8-Multimedia-Wireless-Keyboard/16747373/
p.s. downloads etc runs nicely and the HP micro is super quiet