With respect to the above answers they all miss the obvious question, what exactly do you want to do? Is it stream content from local storage on the box/a local server/nas or are you streaming from sites that would benefit from running a VPN to bypass ISP level blocking or services that require specific things to function e.g. silverlight. For your budget do you need windows and do you need local storage?
If it's a local streamer then a pi2 is a decent shout, at a push it'll deal with a VPN and remote streaming, but it's not ideal so an atom based box at £20-30 more and a Cherry Trail with 4k support comes in at just under £100 would make more sense. The atom is a better bet than the Pi2 as it has the power to run AUO (SAB+CouchPotato+SSickBeard)/rtorrent/chromium on openelec though don't expect to do unRAR+PAR repairs while watching etc. without seriously limiting core usage. If you want an all in one download box then consider an i3 as the extra grunt and option of more RAM (atom has a 2gb limit).
Personally I run a few different boxes, a pi and pi2, an old ION based Revo 3610, a Z3735 based win8 tablet, an i3 NUC and an i3 windows based HTPC. The Revo and NUC run OE and a mix of playback from the local server and online streaming services, the Pi's run OE with access to local server content (kids rooms). The i3 NUC runs a VPN connection for privacy and bypassing ISP level blocking, the windows based HTPC is (or rather was) mainly a download box. I used focus **** to make it boot kodi on startup and keep it to the front screen. I combined this with a flirc and bound koi to the green button (seemed fitting for xbmc as was), this let my Sky remote control my TV, Sky box and HTPC. In hind sight I could do the same with OE now and it would work better, i'd just need a local scratch disk for downloads till they could be moved to network storage.
