You can go to http://www.rackmountnet.com/
and they will sell you a server with pluto pre-installed--they build ours. They can also give you the full specs. There's no license fees, and their prices are about the same as self assembly, plus they will customize it for you, burn-in, etc.
We use SATA RAID, Seagate 7200rpm drives. Audio, like you said, is miniscual--you can run it seems thousands of audio streams simultaneously. DVD's are 7mb/s, so it's the real issue. However a good h/w raid (we use 3Ware) splits the load across all 12 drives, so even with video we've never hit a limit either.