I have a little plastic device called a "Filler Finder" that you can run over a car (has felt on the side to be used) that uses lights to tell you how think bondo is. I bought it around 1985 and used it when I looked at my 68 Firebird just 3 months ago. It still works. A weak magnet may also work.

The car looks pretty nice, but I would have a hard time going over $20. I purchased a 68 hardtop with 61,000 miles and original 400 for less than $20. It was probably a pretty good deal, but I put quite a bit of value on the original engine. This car is a convertible which does add some value.

You need to do what you are comfortable with. If you really like the car and don't care about the nonoriginal engine, a little over $20 may not be so bad. But if the car is not really what you want, you might should keep looking. Don't get in too big a hurry to buy.