Re: new to the board, have a few noob questions i'm sure have been answered.
You can use 24" wheels, you may need to go with a smaller sprocket to obtain the same speed with a 26" wheel.
I used only a coaster brake for years and thousands of miles with no problems. But I will admit it is not the best stopping power. I have since added a front brake as well for better stopping.
I use a atv kill switch on mine, but you can also use a toggle switch.