Lorex was the consumer brand of Dahua, so they are lower end with cheaper materials, less than ideal MP/sensor ratios, and cheaper processors.
Many here started the Lorex route and then started replacing cameras with Empiretech cameras for better quality and could be added to their Lorex NVR. Heck just today I was helping my neighbor replace two Lorex cameras with 2 Empiretech cameras he got during the sale.
The cameras you would want would have AI in them. If you use the NVR for AI, it can cripple the capacity of the NVR by almost half.
Yes you can bring your cameras in like that. They all do not have to go to the POE ports on the back. You will likely have to manually add those within the NVR.
ALL cameras are a security risk and can phone home (china) or elsewhere or be hacked. Best practice is to not give them internet access.
By virtual of them being on the POE ports of the NVR, the NVR acts as a firewall of sorts and puts the cameras on a different subnet.
For the cameras that you would have in front of the NVR, you would want to block them on the router.
What is the goal of the cameras - OVERIVEW or IDENTIFY and if IDENTIFY at what distance? It would be one camera if within 15 feet of the camera and it would be a different camera if you wanted it at the street.
See this thread for the commonly recommended cameras (along with Amazon links) based on distance to IDENTIFY that represent the overall best value in terms of price and performance day and night.
The Importance of Focal Length over MP in camera selection