i'm no expert, but i'm in the same boat so i'll share what i've found so far (i just ditched my car pc and now starting a tablet build)...
i think if you stick with an apple, motorola, or samsung device you'll likely be happier than with the limitations of a Kindle or off-brand device. i explained why i'm going with the XOOM as my platform in my new worklog: http://www.mp3car.com/worklogs/15515...rokee-4x4.html
you've likely got 3 options with sound output: the headphone out (not elegant with regards to wires showing), the dock/charge/usb connector (depending on device), and bluetooth. i've yet to look into what the highest quality bluetooth audio device is, hopefully some people with experience will chime in on this thread. some devices may offer a 4th option, HDMI out. if the devices sends 2 channel audio over the HDMI then you can get a cheap breakout box to pull out the audio, but if it's full surround sound then it's digitally encoded and you need a more expensive "audio extractor" device to pull the sound from the HDMI. i don't see any reason for a head unit except for FM or Sirius/XM.
i think for power, you'll at least want to run the charger off of a switched outlet. you can then use an app like Tasker or a profile manager to tell the device that it should never lock the screen or dim the backlight when charging, but when not charging to go to sleep right away (on Android anyway, not sure on iOS). then most tablets are perfectly capable of running asleep/locked for several days on their own battery power. if it's a long time between uses, you're going to need to leave some way to reach the power button though. there are some Samsung devices that actually run in a weird in-between state whenever they get external power - not quite on all the way, but there was an app or a firmware i saw once that would work with those devices to make them automatically turn on all the way whenever they received power. some Motorola phones automatically turn on when they get power, but i don't think any of their tablets work that way.