Consult a lawyer for the legal side.
There's some excellent advice here on video cameras on a budget.
https://www.youtube.com/results?sear...+budget+camera
Now to the actual content. One of the main things I've noticed about Casting videos is so few seem real. The girls do everything like porn stars so unless they are cast as a porn star, so no need to "test" them, you lose any amateur feel. "Amateurs" fall into one category.
Girls doing it for the money. then sub categories. Girls who are completely up for it, girls who are hesitant then agree, new girls who will not fuck on a casting and probably won't turn up or perform properly on the job. So set out what each model has to do before you shoot her.
Then have other girls at the casting. Make it look like the casting is real with more than one girl arriving. The extra girl has only to watch, masturbate or join in would be a bonus.
Expand on that advice to suit your niche.
Another choice is to have it shot by you with another guy performing, POV or voyeur with "hidden" cameras in the room. Editing 2 cameras with decent software is a doddle these days. Use a clapper board or whistle to synchronise both time codes on the cameras.
Use a single light with a soft box or equivalent for lighting to achieve good lighting an an amateur feel.
Then set the scene with a real casting feel. Explain to the girl what you shoot, how much she can earn, how much work she will get, etc. Then ask her what she will do. This is a great opportunity to tell girls they aren't going to get much work without doing HC. She can immediately go for it or now, make the audience unsure of what a girl will do. This gives you the opportunity to break in a "virgin" arse. While someone's fucking her.
I hope that's enough to ponder on. It was a niche I was planning to do in the UK and shot a few scenes like it.