Will you newbies stop asking these wattage questions before searching? If you had bothered to search, you should have found these links for yourself. Anyway, here are some links so you can figure it out yourself.
As for the display, no we don't run them off the DC-DC. They run off the 12V from the car unless you use a PC LCD, then they usually run off an inverter.
If you are going to use a DC-DC PS, then yes, it is worth it to use the slim dvd, as they draw less current.