I was wondering how cheaply you could go for the video camera route, and you can get a 640×480 camera for
less than £10 which you can talk to over I2C. Output could then be via a Nokia
6100 display, which is 128×128, but possibly a bit too small. It is cheap though, at less than £25. If you wanted something brighter and larger, Cool Components do a
480×272 screen which is 4.3" across, but is £100. Use something like a
Teensy to wire them together, add a battery, and you've got a basic rear view camera system, although you'll still need to find somewhere to mount it that's visible and usable.
Say another £10 for a chunkyish rechargeable battery, another £15 for random components, and you could do it for around £10+£25+£10 ($16)+£10+£15 = £70.
You could have issues if the IO requirements conflict, IIRC the Teensy uses the same pins for I2C and SPI, so if you needed both, you'd have to talk to one of them using software drivers rather than relying on the ease of using the built in hardware and easily available libraries. If one of the interfaces was serial, I think you'd be OK, because the Teensy's serial interface and SPI/I2C pins are separate. Alternatively you could use one of the larger Arduino or clones, which often have significantly more IO than the fairly minimal Teensy.
You ought to be able to send data across between the camera and screen fast enough using SPI and/or I2C, although using a serial link may be a bit more challenging. The Teensy can probably do it, since it'll operate at 16MHz if you run it at 5V (you're not supposed to run them faster than 8MHz is you need to power them at 3V, but they'll often work faster). You could even conceivably run the video stream across to a memory card as well, if you wanted a live video feed for post accident evidence. Talking to a SD card isn't that difficult, although I'm unsure whether you could run video to it fast enough using the SPI link only (all SD cards talk SPI, but they also use other proprietary interfaces, which aren't publicly detailed, to move data around a lot faster).
I've used Teensys quite a bit now, and they are dead easy to program. If you could use the standard libraries to send the data between the two devices, then you would just need to write some code to convert between the data formats of the two devices (and possibly do things like automagic brightness control). That should be a doodle. The Teensy is basically programmed in a variant of C++ called Wiring (if you use the Arduino environment).
Needing a larger screen would bump the price up quite a bit though, unless you can find somewhere that will sell a cheap screen for a lot less than £100 (which I haven't after a cursory Google around).
