[Dave at Pelican Parts]

"Acceptable" depends entirely on your budget, your skills, and the time and effort you want to put into the job. For Chumpcar, I would be tempted to leave all of the rust intact, and make the roll cage the primary structure of the car. That is, if I could weld and had a tubing bender at my disposal. (Safety equipment doesn't count toward the budget, right?)

Tubes can make up for a lot of lost strength.

Patch panels will need to be fabricated or purchased for anything you are fixing. For some areas, you can probably find people selling pieces from cut-up cars, but for the more common structural rust areas, parts cars are vanishingly unlikely to have those intact. So you'd have to purchase new, which will blow your budget almost instantly.

I guess the other answer to "how much rust" is--enough to make the car cheap enough to fit your budget.

--DD