I never buy rust.
Hah hah. Westerners.
Rust on the body would be an issue but who cares about underneath? That's what I pay my Toyota dealer to deal with.
That rust is nothing. You should see the bottom of my truck. Of course, it's spent its entire life in Alaska, Minnesota, and Ohio. It's gone almost 300k miles so far, hopefully 200k more to go.
That being said, if I had the choice to find buy one without chassis rust I would take it. But I wouldn't spend my too short life boiling the ocean to find one.