Are you familiar with Traybuff on youtube? She has done the prayer dress and has great tutorials on the gown and the wig. (has an account here as well). Also, she did the layers of the gown using bias cut circles instead of gathered strips, as it should be. (easier and will help keep fabric cost down).
I also recommend youtuber petrilude for tutes on makeup (coverage and contouring), and covering your brows. Would be great to be able to do the Scarlett arched brow~
There are also human hair brows that would be good once your own brows are shaved or concealed.
If you have any other questions about the prayer gown, please ask. I have studied the gowns for 15 years and am extremely detail conscious. While the prayer gown was not one of my "must haves" from the film, I do have a nice version. It is not completely screen accurate but it also only cost about $150, lol!