People often assume that using a computer as a NVR system is pretty CPU/GPU intensive. "I mean if a gaming computer struggles with 4k games, imagine how much power it must take to record multiple 4k cameras!"
The reality is very different. Streams from digital CCTV cameras are just 1's and 0's. If you are recording the stream direct to disk, then the computer simply takes the 1's and 0's and records them to a hard drive. This takes very little CPU and no GPU power to do this. It does take some RAM, but not high amounts either. Therefore you don't need to be "cutting edge" with any of the computer equipment and honestly you shouldn't buy something that is expensive. Putting that money towards better cameras will be much more beneficial than having a fast computer.
That being said, I think the lowest CPU I would get is a i5/i7 6th generation because that is the oldest generation that allows you to use the CPU chip for some specialized GPU functions (it does effect playback of the material which you will want to do). 8gb is probably enough, 16gb is overkill. I just picked up a Dell Optiplex 7040 (mid size tower) with a i7 6700 chip and 16gb of RAM off EBay for $320 shipped. While that is a decent price, it also isn't the "deal of the century" and can probably be duplicated without much effort.
As far as SSID drives go, it is recommended that you use one for the OS drive. It is not recommended that you use it for CCTV streams to be recorded on. It won't speed the process up and the mechanical drives are designed for this type of use.
So long story short, there is no need to spend $$$$ on a high power computer if you are looking to use it for BI use.