The Duet 3 main board is available to pre-order now, see https://forum.duet3d.com/topic/11540/duet-3-mainboard-6hc-initial-production-run. But it's intended for more demanding applications than yours. You will need at least 7 stepper drivers (4Z, 1X, 1Y and 1 extruder), which is one more than the Duet 3 main board provides. So you will need the 3-stepper expansion board, which exists in Prototype 2 form but isn't in production yet.
OTOH a Duet WiFi or Duet Ethernet + DueX2 (or DueX5 to leave room for further expansion) would be more than adequate for you machine, assuming the 2.4A peak motor current available on the Duet 2 is sufficient for the motors you intend to use. Driving 4 Z motors needs no more CPU time than driving 1, because of the way the code is written.