I considered some of the same issues and settled on a Windows Home Server setup. It has some limitations, but I have 2 HD's in it, and have automatic redundant backup for the files I want backed up that way. In other words, it does a general backups of the PCs on my network, works as music server, and does an additional backup of a copy of my music files that I have stored on a networked PC. So I have 4 copies of the music files: server, another PC, server(WHS) backup, and redundant WHS backup. You can set it up for automated backups, e.g. daily backup if you want it.
It works very well as a music server. Much better and more reliable solution than an NAS. Lots of flexibility, as it is a full fledged PC.
Mine is fanless and running a dual-Atom processor. You don't have to have a powerful CPU for a backup/music server solution, it can be done pretty cheaply on the Hardware side. You can use just about any relatively recent PC, especially if you don't care about issues like low noise or space requirements (standard tower box).
Mine's fanless (heatsinks), external laptop type PS, and in HTPC format. With 2 HD's and operating system it still came in (new) under $800. In a standard box and with fans you could do it for $300-$400, I think.
As far as pre-configured, look at Tranquil PC and dbPoweramp RipNAS. Both are specific "PC as music server/automatic ripper" setups. Configured as low power and very quiet (fanless). A pricer solution, but a good one. The main advantage (or disadvantage) is that they can be setup with Windows or WHS.