You're right about wanting to use a DC-DC power supply.
You can do straight DC power for the laptop with an auto adapter for laptop. However, you'd have to work out a scheme for turning it on & off.
Some laptops may work with a Carnetix regulator, depending on the power requirements. Teh Carnetix also has a startup/shutdown controller to power the laptop on. Many laptops require some soldering to make this happen, though.
A Carnetix wouldn't be the easiest choice if you wanted to retain the portability of the laptop, though. If you plan to gut the laptop, then a Carnetix may be the ticket for you.
As far as power drain, if the engine is running it wouldn't be a big deal with either setup. You can expect the PC to run for over an hour with engine off. Obviously, the lower the power draw of the system, the longer it will run on battery alone.