Spend under £100 on an Amazon Fire TV, Roku or Chromecast and get Plex installed. You may even find you can run Plex from your TV, Xbox One or PS4.
Personally the Amazon Fire / Roku wins it for me because they include a remote and I enjoy using my Harmony all in one to control everything.
Plex, like Kodi, is free so it's worth experimenting with.
Anyway, get it installed on your PC and use it as a server. Further down the line as you start seeing the benefits spend the £300 building a server so you don't have to have your PC on all the time.
In time expand your Amazon Fire TV install base to include you bedrooms, study etc and start living living like a man.
Or, if you're determined to go down the all in one route then then Intel NUCs are probably the way to go. Unless you've got space for a standard size case hidden away then you can probably save money just building with regular components.