Hi Tony,
Crop spraying is not a trivial task, not mentioning only the hardware. You must fly low in areas with potential obtacles. To mention a few, you must control/monitor the amount chemical that will be sprayed. If your chemicals ends in the middle of the mission, you must return to that exact point in order to continue the mission, In other words, you must have a complete system on the hardware and software end. Yes China sells Jiyi crop spraying solutions and I have tested them at our development stage (we use Cube Orange). If you are going to make only one unit, I suggest you to buy one of them since Arducopter has a very little drect support for crop spraying and you must add a lot of functionality yourself in order to be productive.