The key thing to look for in a camera for guerilla documentary production is to be both lightweight and able to get a good image. The idea here is that you will need to be able to move freely with it, take it out and put it away quickly, and be able to maneuver it at the drop of a hat. Look for a camera that weighs less than a few pounds but still retains the features you need. You are going to want the ability to plug in several microphones, have an LCD panel instead of just a viewfinder, and the ability to change lenses.