Have they left the cameras? If so, it would be worth checking their power requirements and video output before you purchase a power supply and DVR. Chances are they're standard analogue 12vdc cams, although 24vac cameras are quite common, as well as HD-SDI, TVI and CVI.
If you need a 12vdc PSU, this would do the job fine: https://www.securitywarehouse.co.uk/catalog/genie-2-amp-12v-dc-switch-mode-power-supply-p-3094.html