saljam@ryanc i don't see "cheap" on the list so i'd say: AXIS M2036. (that's their "affordable" model, but that's only compared to other axis cameras.)
ticks almost all the boxes. the only missing point is firmware isn't fully open source, but it's based on linux and there's ssh and a dev kit for it.
plus, they have decent documentation and seem to be serious about security.
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