I don't think you need an external DAC. That is a fine A/V receiver that you have, and it probably has very good sound. I would want to take advantage of that.
So, I would say just the Pi, and the HiFiBerry DIGI+ board and a good power supply (the better power supplies only cost $3~4 more than the poor ones). Then an optical or coax cable from the HiFiBerry board to the 510, and you are done.
Surely less than $100, or whatever that is in Euros...
