i would recommend in installing cameras without a DVR unless you are willing to sit there and monitor a screen all day. many modern dvrs can send a jpeg to a mobile phone if motion is detected on any of the cameras. if you have a camera inside your home and your system sends an image to your phone..you have enough information to be able to contact the police.
depending where you are, you should be able to have a basic system installed for under $3000 which would include 4 cameras and a dvr.