Oh, wow. Just WOW. Are you located in Asia?
If people are too poor to eat in the fast food establishments, I can see where they could not shop there. Frankly, if you're too poor to eat there, how do you even get there? Public transportation, walking, bicycling? And if your shoppers are that poor, are they the targeted demographic, even?
The client might provide the cameras. Are there laws against recording people without them knowing it? (In some of the states, it's illegal ; in others, it's OK. So you'd have to check.) Here, standard camera gear runs about $500 US Dollars. My first video assignment, the MSC provided the camera and gear, and I did return it to them after completing the assignment.
I truly think this is a case of putting the cart before the horse. This is all research that should have been completed long before accepting this shopping assignment.
I'm not sure if they are available on-line, but there are magazines specifically for restaurant/fast food businesses, and I have read many articles on the efficacies of mystery shopping programs. I would think the information in those articles would be industry wide and useful just about anywhere in the world.
I wish you the best of luck. May you find many perfect shoppers!