Most of the type like mine will take up to a 64Gb SD card, so if you crank down the resolution a bit you'll get more on it. They all seem to continuosly record - that is when they fill the card they overwrite the earliest files and just keep recording. They record to the card and you don't et an option to use an external location, so for the idea of documenting a trip I would just take regular breaks (which is sensible anyway) during which time I'd download to a lappy, format the card and let it carry on. Or if you don't want to do stops too often, have 2 cards and have your passenger (if available) swap cards at a suitable point and download the used one to a lappy as you go.

Test out how long the card you chose will last before you go, by setting the DVR to max resolution and the longest recording chunk size (typically 15min) and seeing how much card space is used per hour during your normal running about.