These are advertised as HD at 720p.  Is that good enough?

It seems like a bargain but why has the price come down $500?  Looks like something is wrong.

I was just looking for an NVR and ran into this ad.  

Could I use it's NVR for my own cameras?  One Dahua 5431  and two Foscams.

https://www.jaycar.com.au/4-channel-network-video-recorder-nvr-with-4-x-720p-hd-cameras/p/QV3012