@DeepCopy: That would be a bad idea in case his VPN decides to go offline or for any reason is not available, then he would be completely locked out.
Typically you want an internal network for the CCTVs which are separated from the internet, and completely isolated from any other network devices. Alternately you could just block everything by using iptables or straight from your router, and only allow certain IP's (like your controller PC) Basically this means you have maybe a router, switch, few laptops, desktop, and none of these can communicate with the cam setup but the one you need. And yes, strong passwords, a DMZ and maybe an IPS/IDS on the network with the cams (but all networks have/should have something similar if running security cameras).
Also note that you should not use common ports for these devices, which should prevent most skiddy attacks.
How many cameras are you trying to setup? I have experience with these things, not the setting up part but the "access" part. And I will tell you that most people have no idea how to really do it the correct way which leads them to getting their systems owned. That includes home and industrial CCTVs.