Canon camera with CHDK as one-button-video-memo-recorder: Which used cameras are best?

Hello.

Ixus and Powershot cameras are available on Ebay from one dollar (auction end price).

I want to build a public video memo recorder.
Unfortunately, with CHDK it does not seem possible to create a recording by pressing and holding a button.
So a stopgap solution is needed.
I thought of putting a mechanical shutter in front of the lens, which the user can open with a lever.
When the lever or button is released, the aperture folds back.

In combination with motion detection, it should only record when someone presses and holds the button or lever.

Because it's video, it should probably be at least 1280x720 at 24p. Better 1080p.
And good sound, better external mic-in.
But just for the mic-in it shouldn't be much more expensive.