Welcome to the board buildfromscratc!
There are a few routes to go regarding power supplies. It really depends on how much power you need, if you want a shutdown/startup controller, and if you want the system to survive cranking.
The main options are:
1. use the mains ps supplied with the case, together with a power invertor (DC-AC).
2. use a DC-DC.
Going down route 1 is the cheapest and simplest. Many opt for this, but you MAY have trouble with the system restarting when cranking.
Route 2 is more expensive, but in my opinion a lot "slicker". You'll have a system that survives cranking, a system that starts up and shuts down safely (when used with a su/sd ctrlr) and probably a system that is more efficient in terms of power usage.
If you're using mini-itx, 2.5" HD and a laptop cd/dvd, a 90W Opus is great. It's small sized and have built in controller.
There are other options too; use the DC-DC ps of a mini-itx case and this
I have no experience with the CarNetix ps, but it's only 60W