Hi,
That’s good. Regarding the CPU you didn’t write what type of USB cams you use and therefore, my remark. If you use a USB cam with 720p resolution an IP cam with the same resolution would use the same CPU load (more or less). Many Ip cams start at 720p and have higher resolutions so just be aware of that.
Dell computers are very good work horses and can take a lot.
IP cams have a lot of built in features, but since you have NCS or Webcamxp it is better to use that as a central. Otherwise you must connect to each camera.
Have a look at this one where you have one computer for the USB cams and one computer for the IP cams. Then connect them to each other and you can have a very nice system. I do this and it works very well. How to connect one NCS server with another NCS server - #13 by Henrik
Good luck,
Henrik