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
I would argue sensor size is what's most impotant to look for in a camera.
Have a look at this thread [1] I have bookmarked. I found it quite informative. I already got some cheap cameras and set them up, but I wish I would have found it earlier. The ones I got are 4MP with 1/3" sensor and perform absolutely terribly in night setting.
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