Some things to consider:
Originally Posted by TobsterUK
The target vehicle(s) will be extensively modified for the project, and I'm hoping the software will eventually override features such as the immobiliser, so an extremely fast boot time will be essential.
If you override the imobiliser you have provided one or potentially two ways for a miscreant to steal the vehicle. this may or may not be a concern right now, but you should think about it.
For fast boot times, tghere are a number of interesting projects going on (moblin? or is that Intel-specific?). If all else fails, consider booting from flash. there may also be mechanical reasons for considering flash to be a better option than a traditional hard drive, but they don't come in mega sizes, which, again, may or may not be an issue.
but don't have a clue where to start with the OS; if I should be downloading and modifying an existing distro or trying to build one from scratch somehow?
Your first question should be "how little do I have
to do?" Ie, what is necessary to meet the requirements?
You should also look at things like emdebian (and any other embedded distros) to see whether, for what you want, it would be beter to start with a minimal, non-desktop, distro and add on, eg, the display driver, or whther you want to start with an orthodox desktop and cut things out.
The other question you should be asking yourself is what you intend to do about power consumption and powering on and off your device. If this is intended for a practical vehicle, you can't just go leaving a device consuming, eg, an amp in a low-ish power mode or the battery will go quickly (maybe you have an extra battery...) and you can't just switch power on and off with the ignition, or you will corrupt the hard disk (/other permanent memory). So what is acceptable in your application that won't kill the battery?