Although I use Apple computers, FYI, the Einscan-S runs fine on a 2.2 Quad core i7 from 2011, which has a separate graphics card, and 16GB RAM. It's obviously a little quicker on the 3.5 i7 desktop with 4 GB graphics card, but there isn't a lot in scanning speed. Nearly all the difference is processing the scans afterwards.
Shining3D recommend USB2, but I just use a USB2 dock in an USB3 port for the scanner. The graphics card element is a little confusing, as long as you can drive a separate monitor in non mirror mode that will do, and nowadays nearly every machine going with an i7 processor or equivalent will have a graphics card better than their minimum recommended.
Down to you really, but quad core i7, discrete graphics card, plenty of RAM should cover it. I boot from an external drive using Thunderbolt tech, but SSDs are coming down in price all the time, and you'll know they make a difference to boot times etc. Check out www.scan.co.uk, and look at their Today only deals... I get a bit of stuff from these guys 'cos they're local, no interest commercially or otherwise.

Good luck
Mike