i7 processor, 5th generation or higher. The common suggestion is Dell Optiplex or HP EliteDesk. Dell runs sales monthly, or if you're patient you can get great deals on eBay from resellers that include 3 year factory warrantys. Stay away from the SFF (small form factor) models as you'll need the large units for multiple HDD to store that much material. You don't want an external graphics card so make sure the process has integrated Intel processing and 16GB ram. I can't say with certainty if you can get away with 8GB, i'm sure someone else here will chime in with a definite answer but i'd recommend 16gb.
SSD drive for the OS and for BI. 120gb is fine, that seems to be the smallest that you can buy nowadays unless you look for less. I have seen 64 but the most common is 120-128gb.
If you're only storing the images that are triggered with alerts you can get away with less space, but if you want continuous recording AND clips and images, you'll need more. I know with BI you can set the alerts folder to a certain size so if you're going the images route, set the New and Stored folders for less and the Alerts folder for more. I'd recommend at least 8TB to start, and if you decide you want to keep more video, add more later.