I see it now. Didn't take me anywhere initially.
So if the existing cameras are all Amcrest cameras, they'll more than likely require the CVI setting on the DVR. I actually tried one of those DVRs to replace an old Swann unit I'd had for years which was an AHD 720 box. None of the cameras came up until I figured out what was going on and changed the settings in the DVR. I was also adding a couple cameras and sort of ran out of horespower in that particular DVR and ended up with an Amcrest instead. Even with 720p cameras the ZOSI will only do 12 frames per second which is basically sub-stream quality. Hook one up and see if you can live with that before committing to it all the way. I think you'll see what I mean in fairly short order. For not a whole lot more money you can snag a refurbed Amcrest AMDV8M8-HD that will blow the doors off the ZOSI and actually provide 30fps up to 5MP -- which is real time streaming. Big difference in both viewing and recording. It will also accommodate IP cameras for a total of 12 channels. $109.00 w/o HDD. It's basically a domestic Dahua XVR. Wouldn't trade mine for the world and also run a Blue Iris server for the majority of my IP cameras.
Anyway, sorry for the sales pitch. The ZOSI may suit your needs perfectly, but the Amcrest takes you to a whole other level for not a lot more money and allows for flexible growth and modernization of the system a little here and a little there if you find yourself wanting to upgrade somewhere along the line -- which you will. And you'd get the hang of the UI pretty fast once you realized what you have on your hands, trust me. LOL
Whichever way you go, good luck. There's always help out here if you need it.