That HD Radio unit is NOT an FM Modulator. It is an FM/HD Radio receiver. Big difference.
I think you know that already based on the other hardware you have spec'd, namely the amplifier.
Also, you have two devices that require audio input to the PC, the Sirius radio and the HD radio.
You will need a device and a plan to switch audio inputs, both hardware and software.
Lastly, a DC-DC PSU is not an inverter, technically. With DC to DC, there's nothing to invert.