Hey there are these small industrial pc that will run off 12V
There may be other cheaper devices around.
Have the device in the boot wire up the speaker out to your line in and Acc power, away you go. You can get windows to auto start then run winamp with my batch file and it loads and starts playing. see http://forums.winamp.com/showthread.php?t=334825
Even better still use wireless broadband and connect to your home pc over the internet...
Your Plan for you usb etc need to be able to connect to the net with out interaction.
Your data plan can be the minimum eg 400 meg $30.00/month because they only slow you down to 64kbs so Im guessing you can run sound a bit rate of 32-40kbs thru your SHOUTcast server Via VPN....I have used a shoutcast over a 56k modem and it has trouble with a 32 kbs stream but 24 kbs is ok
If you are interested in the internet connection I am developing you may want to have a look here
I havent decided to make it a give away yet...Well it needs further testing
You need Winamp pro, shoutcast dsp plugin and Shoutcast DNAS version 1.8.9??
this will turn your IP in to a public station you need to take certain steps to make it Private