Thanks for the suggestions guys but slightlyyyy out of my price range really

I was looking for something cheaper just so my SSDs arent always on with this PC so the GF can watch crap like Pretty Little Liars etc.
How would a wireless HDD work in this situation?
Just get a Raspberry Pie with XBMC installed and a USB Hard Drive. All in one media player with local storage an low running costs.
I think the problem comes from your first post where you suggest you've got multiple locations that you watch content - although it's not clear what devices you've got to do this with?
If you've got one location you want to watch things from then fine, set up a Rasp Pi with Kodi and plug in a USB hard drive for an OK setup on the "cheap".
Just remember that you'll need to buy other Kodi capable boxes for each room you want to watch content from.
The costs just add up when you remember how bare bones a Rasp Pi is:
Pi2 - £25
Memory card - £5
Power adapter - £5
Case - £5
WiFi adapter - £10
IR receiver and remote - £10
External USB HDD Caddy - £15
500GB HDD - £35
£135 total. Obviously this price reduces the more you don't need. Some don't need WiFi, some will make do without a remote and just use a phone app.
Or, you set up a server:
HP ProLiant Gen8 G1610T MicroServer - £185
500GB HHD - £35
Install FreeNAS as an operating system
£220 total.
Coming from many that have done this before, the most cost effective solution (if you've already got the network infrastructure and devices to watch content via Plex from multiple locations) is to spend the money on the hardware to provide the content effectively.
For about £100 more you're getting an completely flexible setup that will form the best backend to support you longer term.
Understandably £100 is a lot to some people though.