First thing to do is read the material in the
WiKi, in the blue bar at the top of the page. Pay a lot of attention to the
Cliff Notes in there. Then, use the search tool, also in the blue bar on the far right, to have a look around this forum for information on the camera(s) you may be interested in or specific questions you may have after reading that material. The current "king of the hill" in the Dahua line is the 5442 series, 4MP, starlight camera series. There are still newer models coming out in this series so it may pay to wait a little before making a final decision. The best way is to buy one, varifocal, in the model line you decide on, mount it on a test rig (bucket full of sand or rocks with a 2x4-8' sticking up to mount the camera on) and checking out proposed locations which will allow you to determine proper focal lengths for a fixed focal length camera.