Well, I took another look at my options, and I'll end up changing some things.
1. Instead of mounting the machine itself into a DIN-slot, I'll have to integrate it into the interior of the dash somewhere. However, my particular model vehicle (2001 Ford Taurus) offers a
1-DIN adapter that also has a slot for a slim drive of some sort - I think I'll see if I can shove a DVD-ROM in there. Basically, the idea is that once the machine is placed and
correctly wired, I won't have to screw around with it again... so I'm willing to live with a pain-in-the-arse level of access to the machine itself.
2. Due to this suddenly relaxed space requirement, I may well go for a dual-core solution with some sort of 3D video card (maybe an old GeForce4 or something - anything's better than an S3 of any kind).
3. Reading more about flash, I think I'll run the flash drive in read-only mode, running the OS from a RAM drive. I
should be able to keep the /usr partition on flash rather than RAM (as nothing should write there), which would save a large amount of RAM...