Is this 68 Dart worth fixing?

By roll down rear window i assume you mean the qtr. glass.
This is a hardtop i think they are more common than a post car(stationary qtr. glass).

Engine's sizes ranged from /6 to Hemi's.
The most common would be /6 and 318's the rarest would be the Hemi.
There are several member's on the board with 68 and 69 Darts.

You have never seen a 68 Scamp because they weren't built until 70/71.

Hope this helps. :)