1 - Each frontend has it's own system requirements. Your system specs should be hable to handle all the major front-ends with no problem, though.
2 - Look online for a power calculator to get a good estimate.
3 - Many people feel the extra cost is worth it for sunlight-readability.
4 - Personally, I think the felxibility of being able to upgrade the CPU at some point is a plus. But it's really 6 of one and a half-dozen of the other.
5 - Yeah.
Avoid the M3.
You don't need to spend more on an automotive-grade HDD.
For aesthetics, a slimline slot-load drive is ideal.
Mitch (A member here that goes by MJS, I believe) offers solutions for the Sirius integration w/ the PC.
And let your passengers watch movies that aren't quite so odd. I'm sure they'll appreciate it.
Looks liek you've done your homework, which is refreshing for a new member. Most newbies don't.