Hi HerbS,
I'm planning similar unit. I read you should not have HDD and DAC both on USB. Do you have a concern here?
No, because:
- Ethernet on the Pi also is connected to the internal USB hub
- WiFi is also connected to USB
- may only be an issue with very high sample rates.
I think there is no difference in playing files from a NAS or local HDD to your USB DAC.
I personally prefer I2S DACs, because they are cheap and sound great.
With I2S DACs i never had issues in playing highres files.
On my ODROID i can play files up to 32/384kHz without stuttering.
Frank