My gut instinct for a machine like that with direct drive steppers, would be something in the 4Nm range to give plenty safety margin.

I think personally I'd consider altering things to use a belt drive with some Nema23 steppers, which should give you better performance. With big Nema34 motors, for best performance you've really got to use high voltage drivers, but you're then still limited at torque drops of quite quickly with big steppers.