Like vitrocz said, the roof panel is NOT structural outside of the fact it's connected to other things that ARE structural. There are braces beneath the roof that do the work.
The repair will not cost enough to total the car, either.
Where are you guys getting this stuff from??
As far as body shops v dealer...that's a tough one, because Chevy will warranty the work done at a dealer - they won't do the same if taken to a body shop. That is, unless the dealers in the area are not equipped to do the repair, or some sort of special deal/contract can be worked out. Luckily around here, there's a dealer with a full-fledged body shop that isn't horrible.