The extra parts involve a voltage regulator and an isolator.
The regulator is an LM-317 which is a simple adjustable regulator. You could probably just form a simple voltage divider circuit instead to save yourself some parts.
The "inverted PSON signal" just means "standard ATX psus power on if the PSON line is low, apparently the Compaq PSU is the exact opposite." Because of this, the motherboard would output a high signal all just to turn on, and a low at any other time. Any standard PSU would see the low as a signal to turn on, and therefore, it would always be on.
The hardwiring of the PSON signal would mean that your motherboard could never turn off your power supply (I think). Your power supply would never be in standby mode and waiting for the PSON signal to be sent from the motherboard, instead, it would always be on with the fans blowing and etc. For a car application, it doesn't seem like too big a deal...