I have been passively looking at doing something like you are suggesting, but then also turning it into more of a video camera. My main motivation is that it seems pretty much impossible to get a off the shelf sensor that has DPAF.
One big hurdle getting the datasheet for the sensors - they are usually quite quirky and even if you get the datasheet and all documentation you will still need support from the manufacturer to get everything working reliably.
I think the issue would be the missing datasheet. But I was thinking the same, old cameras seems to be the easiest source of a good sensor.