There are power calculators out there that can give you an approximate power draw for an opetical drive. The general recommendation is a slip laptop-style drive, as they only draw power from the 5v rail, while a full-sized desktop-style drive pulls from botht he 5v and 12v rail. Also, slot-load drives are far more convenient than tray-load drives.
Yes, the cables from a DC-DC PSU are the same as from an AC PSU, although you're going to have fewer HDD molex power conenctors.
You can figure out what PSU will work best for your hardware. There's a handy FAQ here:
http://www.mp3car.com/vbulletin/advf...iew&advfaqid=6
As far as LCD power, it depends on the LCD as well as how you want it to behave. Some power theirs from the PC PSU. Others straight from the car wiring.