First  question is are they pan/tilt  zoom or fixed camera's?  Leave a pan/tilt pointed into the direction of the morning sun from when it was dark the night before, and bingo- maintenance time.  Second question- is there a local somebody who is factory trained at repairs to that brand in your area?.  Third question- are you allowed downtime?  Does the customer keep spare parts on hand or are you going to have to?  It's been a few years since I had a contract for maintaining of several hundred cctv cameras that got installed under my contract, but give me some time and I will keep remembering all the factors that you can run into..