The GT and its derivatives should be the ones with the most appeal to collectors in the future. The GT badge is one of the most iconic of all in Australia, so that should be a no-brainer. Look for a one-owner car, make sure it's a numbers-matching example, and go for the classic colours, with stripes where they were optional. I don't think the interior trim material will make much difference. As for the odometer, go for the lowest possible.