As far as the condition, I like the look of the trunk floor and rear edge of trunk lid. Both are an indication that the area around the rear window might not be too bad. As long as you are OK with a vinyl top car, this car might be worth the money.
As far as number of 1970 convertibles made, it was between 1900 and 2000. A general rule of thumb I've seen is that you can't expect more than 1/3 of those to still be around. For 1970 convertibles, my guess is around 600 are still on the road. I can also say that one is definitely on the road because I own it.