That looks about like my 80....
These cars are well known for their excellent durable undercoating....which remains in place even when the metal no longer is. You need to do some good prodding around under the car with a screwdriver and see where the soft spots and poke-thru spots are, with rust like that there's bound to be more hidden.

After that, look at what it's going to cost to fix it right, compare that to the value of the car, and also consider what you could find if you look around a little for a good solid car say with a bad engine, which could be grabbed on the cheap, that you could swap your good parts into. It's not exactly like theres sentimental value in it because it was his grandma's first car, its just an old car he picked up.

My car has a big hole like that in it, rust bubbles oozing out of the doors and door frames, under the body the suspension parts are all heavily pitted, soft and cracked looking spots in the undercoat, and every visible welded seam has bubbling around it with that familiar orange stain oozing out. I'm not even going to try fixing that, it's a lost cause and financial moneypit on a car that simply is not worth that much any more even in excellent shape. I went and found another identical good car with a very well preserved body inside and out, and dead transmission, for $100....

__________________
Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale