The cheapest (~15 USD bullet, 20 USD dome) PoE cameras on AliExpress (focal length is pretty much the most important parameter to look at, depending on the fov you want) hooked up to a Unifi NVR. Skip all the vendor manuals, setup steps, and apps - adopt them directly to Unifi Protect.

I put them on separate vlan where they get no outbound network connectivity.

For cases where you want things like facial detection or license plate detection (automatic doors/gates) get a Unifi AI though and those things cost, but for normal perimeter/room monitoring the cheap ones are very good