If you find a good installer, getting behind the air handlers or ductwork should not be a problem. There are tips that can shoot foam up to 5 or 6 feet pretty accurately (long shot tips).
With the amount of time that you would be running your air conditioner in SC you really want to use foam in your roof rafters.
- It will dramatically decrease the load on the air handlers, which saves you money on your electric bills, and you are not fighting the 120-degree temperatures in the attic along with the humidity and condensation that comes with that.
- It's that condensation that will lead to mold and mildew in the attic.
In your neck of the woods, I would recommend about 7-8" of open cell spray foam or 4-5" of closed cell if you don't mind the added cost of closed cell.
Hope that helps.
For more information:
Read "What is the safest and most effective insulation for our attic crawl space?" a Q&A answered by Ian MacLeod.