This one has probably popped up a number of times, but I just need help finalising hardware.

I have a Nexus 7 running Timur's USB ROM, and a USB OTG cable which splits into multiple inputs (I will be hooking up a hard drive, DAC, and power for now)

Mounting is fine, I can get that covered. I just need help with the rest of the hardware. This will completely replace the existing audio head unit (it's already been ripped out).

I assume this is what I need:

USB DAC for audio > 4 channel amp > Speakers
I will need to power the Nexus as well as peripherals, so will I need a DC-DC power supply? Does this plug straight into the cars battery?

Any help will be greatly appreciated!