If you've calculated that you need 265W, then yes your best bet is the opus you linked. Almost all dc-dc car power supplies, including the opus, are "intelligent". This means that they will power on with the car, and shutdown the computer properly when you turn off the car.

Almost all dc-dc car supplies are more expensive then their 'home' counterparts because of the "intelligent" feature, and the compact designs. If you want to spend under $150 for a car power supply, its possible -- but you'll have to change your computer components so that it doesn't require so much wattage. Hence why VIA boards are so popular =)