Don't. Riveted joints flex unless properly engineered and executed. That flex allows crud to enter and you'll have the same or worse rust in a couple years.
That car is not going to be a cheap, fun, budget friendly cruiser anytime soon. Sadly, you wound up with a car that needs some fairly extensive work. The fact you need to ask how hard it is to repair suggests you don't have the tools and knowledge right now to execute a repair. Trying to do it cheap means you're going to do it twice. The first time will cost you more than you realize and doing it right the second time will cost more because of the "cheap" first try. I've been there too many times..
A second car that's more solid would make better sense for a fun budget oriented car.
