I have your contemplated system, made some mistakes, and am redoing with a new computer.
What I did 6 years ago, is have a rack mount CPU with 3 500g hard drives in a RAID formation so if one fails the data is mirrored on the other two. I use Media Monkey as my juke box. I have a Layla Soundcard into the tape inputs of a rack mounted Adcom set up, with two huge amps, preamp, and sequencer. I added a rack mount battery back up and lights. It is really ugly and with all the fans, is very loud.
What I would do now is have a similar, but remote, rack mounted system, but now understand that you can fit three 1TB drives into a rack mounted CPU now. Alternatively, and this is what I'm planning is to buy a a standard 500g rack mounted computer and CPU with 3-4 external Lacie HDs, probably 4 TB each, one main, one back up, and one third back up, all formatted to back up with Folder Clone. I'd have a network card, and send the RJ45 to the music room. In the music room, I'd have a cheap computer with a sound card to my analog system, probably rack mounted.
Thats what I'd do now.