Rear quarters outer panels aren't replaceable as such =( quality panel beaters could cut/de spot-weld ) but huge job.
Is it repairable =yes !!!
Will it be worth it dollar wise ,only you can say!!!
Going off that 1st photo ( That's a big rust hole ), I'd be surprised if there's not more rust damage in boot floor , wheel arch and rear panel wells, e.g. = where inner/outer guards meet.
Will need out at least: rear window and trim , rear parcel shelf , rear seat , boot trims and seals , fuel tank and filler ( unless you want to risk blowing up welder/fabricator.
You'll not want an normal modern insurance panel beater and that's the challenge = find an actual metal worker /panel beater /fabricator !!!