I suggest not using the picoPSU because it requires you to also purchase a regulator to feed it stable 12v. For the cost of botht he picoPSU and a suitable regulator, you can get a full-featured DC ATX PSU (M2ATX, for example) that will regulate power, has a built-in startup controller, timed shutdown, etc.
1GB RAM is excessive in the car and will slow down the POST, meaning a longer wait when booting cold or resuming from hibernation.
Most people prefer a slot-load laptop drive because the trays in slimline tray-load drives are fragile. It's very likely you could snap the tray while loading a disc if you hit a bump or pothole. Also, a slot-load is a lot easier to integrate into a dash or some other location.