I don't know who gave you those quotes, but they're probably smokin' crack.
You mentioned that the "roof" is going and almost gone......if that's the case, I can't imagine what the rest of the car must be like in the problem areas......You can purchase a Dart roller in new paint for 4 to 7 grand depending on the quality and additional parts on it.......13K my foot.......they just want to take you to the cleaners but good.
I had an estimate drawn for my 68 Charger to finish blocking the car, then epoxy, finish block and paint base/clear in a good shop up in NY.......Came in at about $5,000. for the whole job.
If your car is that rusted that you're honestly worried about it, and it sounds like you are, your best investment would be to find another donor Dart body with some decent panels, and no rot like you're describing. No joke, I've seen some solid cars sell for $500. and up to $1500. that would solve all of your problems...........
SHOP around.