I assume this is the correct motherboard? I believe that´s the standard 24pin ATX molex, so the usual standard of an opus, mini-box, or the other usually purchased automotive PSU´s will do. To know which one you´d probably want would be to do minor calculations of everything that´s going to draw it´s power from it. Motherboard, CPU, RAM, HDD´s, ODD´s, etc.. Then search here for threads on it, be it the M4-atx, m2-atx, or whatever and decide from there. The FAQ has some more relevant info on how to choose.
WLAN is just as easily done with a usb one, in some cases even better since you can run a decent length of usb cable to put the attached wireless usb adapter in a place that might have better reception.
If you want a drive that you don´t have to worry about dieing eventually due to vibration, g-force damage from potholes and the like, go with an SSD, some great sandforce controller ones for the $100 mark (US).
For optical drives (slim line), from the articles I´ve seen the slot loading ones seem to be more durable for this application then the tray, and of course easier to just mount to the underside of a seat and throw in a cd when wanted.
For the audio, you will have to adapt the output, usually 3.5mm stereo plugs on the back of the PC to the amp(s) you wish to use. This can easily be done with the 3.5mm male to male RCA cables or adapters. Shielded. Noise from the car, the pc in the car, and the PSU are all going to be your enemy, be it RFI or ground loops.
My opinion is that your bits are ok, but if your going to need to sink X-amount into it to get it functional, you may wish to view some mini-atx boards that might just give you the bits that are missing for around the same price, be less power hungry, and be easier to throw into the car. But that´s just my take on it.