Whether you replace the roof completely or straighten the original is going to have to be a personal decision.
Just about every cab has dents in the roof, and working them out is quite an exercise. I'm still in the middle of that myself.
My experience with repop parts (even something that should be as simple as cab corners) tells me that a replacement roof isn't going to fit exactly right and will need quite a bit of work to get it right, especially with a butt joint required at the rear (the replacement roofs that I've seen don't include the rear window area.) That and the interior panel at the rear window will need to come off to gain access for a butt weld to be done properly.
But even if your body shop isn't capable of getting the dents out, you could give it a try yourself and be an expert by the time you're done, including metal shrinking.
