It's either a motor with a integrated leadscrew need about 420mm, found one here
or a normal Nema 17 with a coupler and a leadscrew.
What gives a better print quality?
Prusa said this at the MK2 announcement:
What no Z-Hop possible without a integrated leadscrew?Integrated leadscrew Z axis
You can see a lot of clones using trapezoidal threads for Z axis but they always have one big problem. Cheap, springy, spongy coupler which introduces wobble into the print. From my experience M5 threaded rod with flexible tube coupling works better. But there is one way to improve – ditch the coupling. We ended up having our custom motors manufactured with integrated trapezoidal lead screws. This means no coupling, no wobble and awesome layer alignment!
This way Z axis got also way faster and more wear resistant which leads to possibility of using Z Lift where the nozzle lifts up on travel moves and prevents scarring (leaving unpleasant mark on already finished layer, by dragging the nozzle on top of it). The i3 MK2 is an ideal platform for this as the extruder is quite small and won’t wobble compared to boxed printers which would have to quickly lower and raise a heavy print bed in Z axis to achieve something similar but not quite as the object moves the whole time as well causing further issues.