Hi, and welcome to AC!
I'm a newbie at computer servers, too, so I'm no use on the motherboard/chip question. Sorry...
Your server case is beautiful. Great choice!
The PPA USB card is worth the effort. Kai-Yu (Paul) Pang is great to work with and his card is worth the very low price. Get a nice linear power supply (5v/2amp min) with a 4-pin molex connector for the best sound results if you can. Same thing for the motherboard (linear power supply, voltage/amps depending on final selection). I'm not sure how that works with a Pico DC-DC input.
Also, give the PPA 200+ hours for best sound. It will vary like a capacitor burning in between startup and 200 hours. I set my computer on "all songs" when I would go to work to get an extra 10+ hours a day on it (amp off).
Please consider adding Fidelizer 3.0 to your software setup. I found it makes a huge difference with two channel (not tried with HT). Give it a try...it is free and non-permanent (need to run after each reboot).
Enjoy...
Best,
John