Hey Looseworld,
An HD webcam is going to be your best bet. Check out the Logitech HD Pro Webcam C910. It has great image quality, solid audio, and has been tested extensively with Livestream:
http://www.logitech.com/en-us/webcam...s/devices/6816
Since you plan on mounting the cameras up high, I recommend coming equipped with USB extension cables.
You can connect multiple cameras to each computer (providing the computer meets the minimum system requirements and has available ports).
Jason