Flurin,
I've been in the same boat for quite a while but I've got a link here that might interest you (German, longer read ... take your time).
Until I read that, I was quite sure that it would take tons of money to do what I want to do (especially, to be independent of streaming etc. ... no time to explain right now why).
In short: This guy bought himself a cheapo Netbook (one whose fan could be shut down 99% of the time without ruining anything), hooked it up to a (realtively) cheap but excellent (!) DAC (Digital Audio Converter) and then hooked his external drive(s) into the Netbook (which runs foobar).
Bingo!
Done.
Uhm, yeah, but that's old news, isn't it?
I still prefer the Ampache way (mentioned that one earlier this year here) because, IMO, it's a waste to use a netbook that runs a file or web server and use it stationary. I'd rather use the netbook as portable frontend.