Based on the distros I have installed and conducted some testing on, the easiest distros are Linux Mint and Manjaro. Presumably, any distro which uses LightDM should work out of the box but I will admit that I did not try every single Linux distro out there. I also had a lot of success using most distros if I picked the MATE version as the starting point, such as Fedora.
Additionally, while I did not try starting out with a Gnome 3 based distro and replace GDM3 with LightDM, I did try starting out with a KDE-based distro and switch to LightDM and it did work at creating multiple seats. This was on OpenSUSE using YaST.
The display manager is one of the most important pieces in this process, which means that LightDM and MATE's GDM2 are your best bet for a place to start. This will allow you to see the software in action and test out your hardware, so that you can confirm that you don't have any incompatibilities preventing you from establishing your own multi-seat computer.
(For reference, as of this writing the versions were Linux Mint 18.x Cinnamon and Mate, Manjaro XFCE 17.xx, OpenSuse 42.3, and Fedora 26/27. )
EasySeats is open source software developed to bring ease to users who choose to go multi-seat. It supports video cards for seat creation as well as external USB docks.