I've done a lot of 'glass car paint over the years. I've worked on really nice 'glass, and really bad 'glass. There are a few things I've decided are the minimum requirement. A body that has enough thickness to maintain a decent surface, any extra panels have to fit within reason, and no more than a few dozen pinholes. Anything else is useable.

I haven't seen any bodies that are paint ready when they come out of the molds. There is always some shinkage, especially where inner panels are bonded. Other issues involve the condition of the original "mold plug" body, curing speed, which can cause warping, or parts that aren't completely cured, and the condition of the molds.

If I had to choose the most important thing, it is thickness of 1/8th inch or more. You can't overcome that, if it isn't there.

On the other hand, I prefer 'glass over rusty steel, if I were building a "keeper". Once it's done, I wouldn't worry about old problems resurfacing later.