Take a deep breath. Don't buy anything yet.
Start reading here: New to CarPCs? Don't know where to start?
Then do some more reading here: http://www.mp3car.com/vbulletin/the-faq-emporium/
Then, if you're still fuzzy about the whole power supply issue, read this: FAQ: Will <insert PSU model> Power Supply work with my system?
One key to choosing a DC-DC PSU is whether the motherboard is truly ATX-compliant. A lot of miniPCs like you linked to are not. The M1ATX is for ATX-compliant boards only. There are other PSUs like the Carnetix that would work with non-ATX setups.