@graesand, RepRapFirmware should already support that kinematics. You will need to create the UVAB axes using M584, and use M669 to specify Cartesian (or CoreXY) kinematics, but provide a custom matrix to specify how XYUV map to the corresponding motors.
You will need to do the preview GUI yourself. You may be able to adapt the existing GCode preview plugin in Duet Web Control.
For the hardware, you will need 7 motor drivers assuming a single Z motor. If you want to use Nema23 motors then I suggest you use a Duet 3 MB6HC as the base board. To drive the 7th motor you can use a Tool Board if it is a Nema 17 with modest current requirement; otherwise EXP3HC. If you decide that Nema 17 motors are sufficient, then a Duet 3 Mini with the Mini2+ expansion board should suffice.