1. some people quite happily run of 60W supplies it depends what you plan to run.
(read up on some completed projects and see what people run and also read the STICKIES) Some people even run P4 computers off a 150W Opus.
2. Alternator will be fine unless you also plan on using a lot of hight powered audio amps etc. 150W PSU (for example) is like running your headlights. It's only 12A and look how many 10 and 15A fuses are in your cars fusebox.
(i suggest you do a search on 'alternator' and 'load' etc)
3. Discussed a lot - a PC can read CDs that my home stereo can't, why? because off error corection and reading in advance and caching stuff. How many DVD players are there out for rear passengers? Loads! and they are not even based on PCs. Think about it!
Oh and it is illegal for you to watch DVDs when driving anyway.