I'm using a Nexus 7 2012 the following way
Nexus 7 2012 loaded with CM10.1 Timur's USB ROM and Kernel
MiniBox USB-DCDC-200 power supplier tuned to 5.4v output to the y-OTG cable
D-Link USB hub DAB-7
HiFiMeDiy USB Sabre DAC/Converter
Kingston 240GB SSD hard drive
As for ROMs -
The 2012 version of the Nexus is better to use than the 2013, has more car dedicated ROMs for example Autodroid over SlimKat
Most of the issues with most other tablets and smartphone is fast charging in fact - you may find a good working solution for USB audio but in most cases you will find an issue with how to charge the device fast enough.
For some devices wireless charging is possible but is it fast enough?
The PSU I have can be output programmed so I've programmed it to output 5.4v and it charges the nexus fine, other possibility is to set the DCDC-200 with OFF timer so after a drive it keeps charging the tablet if needed.
But - if you install a fix installed and you do not intend to extract the tablet from the car each time - then take out the battery from the first place!!! it is a ticking bomb!
What you do instead is you buy two power suppliers:
1. 4.2v connected directly instead of the battery to the HU board.
2. 5V connected to the YOTG cable
As for sound quality - I use the Sabre DAC as a converter to optical but if you use analog 3.5 PL jack than the sabre DAC has an excellent sound quality.
I use Mortplayer due to its usability - awesome app!
I use USB Audio Player Pro for maximum quality - I've written a lot about it.
I also use the tablet for OBD fault scanning using Torque and a BT ELM323 scanner.
As for Radio - I use WiFi threading with my Iphone and apps like - 8tracks, TuneIn Radio
As for FM radio I use SDR Touch and a DVB-T dongle that has the required chip in it (in the apps site page you will find a list)
As for reverse camera - currently do not need it as I have a dedicated screen for that but there are good easy solutions for that using UVC based video grabbers (easy cap does not work on many ROMs) and a patch USB cable with a relay.
To put it short - if you are not using a fixed install than I would go for a smartphone with Android L or something the native supports USB Audio such as Samsung Galaxy series or LG G2/G3 or smartphones with Android L
If you are using a fixed install and do not wish to remove battery - worry about fast charging
But I would really recommend to remove the battery if it is a fixed install - due to this:
This is a GPS device with 1000Mah battery exploded in a car
Now - the nexus 7 has a 4500Mah battery!!!
Just type in google images "old gps battery explodes" and see what happens if something of that sort happens.
Hope I helped.