I will set you on your way:
PSU - keep it within the power capabilities of a dc-dc. go for the DS-ATX 220W and you will be able to build a very nice system on that power budget.
as for a case - dont bother. they cost money you can spend elsewhere, they restrict what you can fit in and you can build a much more integrated system with a custom made enclosure which you can also deisgn to suit your car and provide a better cooling design that is not compromised by how it is mounted, bits of the car etc etc.