An i5 CPU should be able to run 6x 1080p cameras at whatever frame rates you want, as long as you've configured the system according to
Optimizing Blue Iris's CPU Usage | IP Cam Talk
I suggest at least a 6 TB hard drive. Assuming you have the cameras encode at 6 Mbps each, it will take 4 TB to save 2 streams for 1 month. (
12 Mbps * 1 month - Google Search) The remaining space can be used for motion recordings from the other cameras. You can set up two storage directories in
Blue Iris both on the same hard drive but with different size limits. Set your two constant recording cameras to one storage directory, and set the remaining cameras to the other directory. This way your motion recordings have a dedicated amount of space and can last longer than a month.