i'm using a microATX board w/ an ITPS controlling an invertor via relay. my 3.5" HD hasnt had an problems so far, its encased in one of those removable racks with an in built fan.
the ITPS simply controls turning the PC on/off as well as its power supply. checkout: http://www.mp3car.com/products/itps.html
My ITPS is connected to a relay to switch the power for the invertor. My motherboard is set to turn on on recieving AC power, and the ITPS turns it off when i turn the ignition off.
As for your screen, the ITPS voltage regulation is poor, u'll need to run a 12v rail from your power supply (whether DC-DC or using an invertor) as this will produce a far more stable and suitable 12v for your LCD.