Rusty brake nuts & cables: Should I change my brakes or not?
Surface corrosion on nuts and bolts isn't a safety issue, so you don't have to replace the brake calipers. As David D suggests you can clan this off with steel wool. I would carefully apply a thin coat of grease as I spraying WD-40 on bikes attracts crud and you certainly do not want to get it on rims or brake pads.

Rusted cables is another matter. If the cable is rusted the housing probably is too. The corrosion may impede the travel of the cable through the housing and affect braking performance. I'd replace the cables at least with good quality stainless steel ones. They are not too expensive. You probably should have a look at your shift cables too.