PMd heartworm
hi there mate,
answers to your Qs:
wireless no probs, you can easily use a USB wireless adapter (i did have one connected to it whilst i was using it but have kept hold of this for another PC)
i think the spare wheel would be fine but as long as it was well cushioned - i think the key would be not to have it bashing around in there, perhaps seated on a foam base or a flexible piece of wood? the fan is on the top of the case so it would require ventilation at the top to ensure it stays nice & cool
sat nav, again this is no problem, you just need the right software (i'm not sure which one you'd choose but this is a common application to run on the car PC). what you would need is a USB GPS mouse to go with it. there's one that i'm giving away with the PC but i couldn't get it to work hence why it's free as i can't be sure if it works
as for sound, you would either need a head unit with a line in connection for you to connect the output of the sound card directly into there or you would need an amp to connect the sound card to & then run your speakers through the amp instead of through a head unit