Out of the box idea: do some quick patching, sell it when it’s pretty, buy a less rusty coupe. I bought one for under $20k that has as little rust in those infamous areas as I’ve ever seen, the front inner fender pockets are quite literally dry and smooth. The fuel tank surround is rock solid top and bottom, areas above fuse box and glove compartment are clean and no filler in the nose or tail. It 100% HAS RUST, just not where I can find it. It was reasonably priced because it isn’t pretty. The paint looks old, the interior leather was dried and cracked and the orig motor was a bit worn out. The car had no showroom appeal. But I think I can address all these problems for less that it would cost to fix a rusty but pretty coupe. I’ll bet there are still cars like this to be had if you look hard and long enough. It took me a 18 months and $1000 in professional inspections for cars Located far away.
Step One: buy an electronic film thickness meter ($300). I have an Elcometer for steel substrates and an Electro-Physik ($250) for aluminum bodies.
Step Two: find a knowledgeable friend who can walk you thru the common trouble spots.
Step Three: no matter how pretty it looks, check the paint thickness everywhere, with attention to areas you learned about in step two.
Step One/Two/Three alternative: have an E9 specialist inspect the car for you. Best $350 you’ll ever spend.
Step Four: Ask the forum for an opinion.
Step Five: make a suitable offer.
Following this method I’ve yet to buy a lemon but I have missed out on a couple spectacular deals ( good car, stupid offer by me).
My coupe measures: 6-8mils over 95%, 12-13mils where the body panels are welded together at the factory. 25-35mils where Mario deleted the side markers and did various quickly touch ups. Where you had 1/4” in the nose ... mine is 7 mils of paint, which is OEM thickness. And I knew this before I made an offer. That’s why I know there’s no significant filler.