These are all good options. It all depends on how you want to develop you carpc in the future.
Option 1: Good because it would be cheap. You could still use the shutdown controllers sold here but it depends on how you power your pc. Downside, you may not be able to do all the things you want in the future.
Option 2: Good way to go. small size but costs more money. I personally don't think the via boards are quick enough, but it all comes down to what you want to do in your car.
Option 3: More expensive and a larger pc install, but gives a lot of room for expansion if you buy a reasonably quick machine. you can also run things like flash skins to make you r setup have that wow factor.
Mainly this hobby stops with money. The more money you have the more options you have, ie better soundcards, screens, quicker processors and the like.
For answers to a lot of your questions, have a look at the wiki. (link at the top of the page) or the FAQ emporium. There is a lot of info there. You have to sift through some crap, but you will find the answers that you are looking for.