Do a water test: spray a hose on the various parts of the back of the car, starting from the bottom and working up, uuntil you get water in the car. Don't have the hose nozzle set to a hard stream, but more of a spray. As has been said, the taillights have gaskets, but so do the mounting pins for the body mid-belt molding, the side amrker lights, the hatch (of course), and the spoiler pedestals and side pieces. All the gaskets and seals are readily available from Toyota.
The rust in the tire well can be removed by sanding, sandblasting, etc, and then the bare metal can be coated with POR-15 to prevent further rust.