Secondhand ND5 XS £1600
Synology 216j or 218j £180
WD Red or Segate NAS drive 3TB £100
Netgear GS105 £25
Ethernet Cables £12
DBpoweramp (software) £25 or EAC (whatever you want to pay)
Total £1942 - i.e. in Budget!
Provided you want the NAS for Music streaming and backup/general file storage a Synology 216j (or 118 or 218) will be perfectly adequate. The only limitations you may encounter are with transcoding, but the simple answer is to just use WAVE files, the more complex answer is to use Minim Server). The more powerful NAS drives are intended for use with multiple concurrent users (mote than 10) or for intensive jobs like transcoding video files; for music and domestic use, they simply aren't justified.
Or if you want to go even cheaper then google for Volumio. They’ll do you a pre-built Pi based player/streamer/renderer/whatever for just over £100. use with the rest of Huge’s kit and it’ll get you started for less than a grand!
And it does internet radio as well.
Also, if you have your own DAC then Volumio will do you a digital streamer.