@usverg: I've set up an old Version-1-256MB-raspberry nearly same way as you described. Before there was also a little "click" on every write-access to SD-card.
Solution: The system is stored at a read-only mounted SD-card. '/var/' lives in RAM only. The music is stored on an USB. database and statefile and so on came from USB too (other partition), but will be copied to RAM during MPD-start. During MPD-stop (this is the only moment, when something is mounted read-write) database and statefile and so on goes back to USB. '/etc/init.d/mpd' was modified to realize all these things.
For safe shutdown I've included a NiMH-battery and some scripts/binaries, so I can simply cut the power.
Soundcard is an UCA202 from Behringer (PCM2704 based?).
I've running this since more than 7 months, the kernel-version and all related things are a little bit older (end of 2012?). Since I'm not sure, if it would make problems, to switch to newer versions, I see no reason to do so at this point. But if wished I can search and post all related things, they should also work on newer versions.
I know, this is no audiophile solution, but works fine for me and I can not hear any difference between real CD-Player and this. More I didn't need ...
PS: An important hardware-related thing on such such setups is: AVOID ANY GROUND LOOPS! It's much better as using a ground-loop-isolators after all ...