I'm only using ABS because it came free with the printer and I only have a partially full spool of PLA .
I just figured a small, enclosed printer with a z axis bed and a hotbed capable of reaching 110+ degrees would work considering some people print ABS on open frame i3s. I guess I was wrong.

As for what the center piece is, there are 6 components in the center assembly so I'll try and explain them all without a massive text wall. The two identical big grey parts are the sliders/bearings for the XY junction, the two red plates mounted to them (bottom and middle) are what clamps onto and holds the Z axis tubes. The top red plate is a mount for the Z axis motor (motor shaft points down and couples to a leadscrew that runs through the two other red plates). The grey block in between the two z rails and red plates is the Z carriage which slides along the two z rails, it's driven by a leadscrew which runs through it via two captive nuts, it has four mounting holes for interchangeable toolheads.

If you can tell me the part name (the module it's made from, they're listed at the bottom of the .scad file above the echos for the BOM) I can explain a bit more about the specific part you're interested in.