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The nema17's produce a fair bit of torque. Depending on your drive mechanism (again I can't see your design at the moment) I would think they would be fine to make everything move. HOWEVER, bear in mind that with weight comes inertia which can kill your print quality. The reason for bowden extruders is to lighten up the print head as much as possible. This allows for faster movement, greater accel and decel, and quicker printing. The lack of inertia allows for this. Lack of inertia also allows for repeatable positioning though because with greater inertia comes the need for greater torque to overcome it. If the motor doesn't have the torque to decel or accel the way the software is telling it to it's always going to be "behind" so to speak and will be constantly trying to compensate. Weight on the Z axis isn't as big a deal because it's not constantly in motion, just the occasional step to lower the platform between layers. X and Z though should be as lightweight as possible for best results.

Regarding engraving, I'm not too experienced with it but engraving typically does not require a great deal of force since you're usually talking about a high speed tool removing small amounts of material. Milling on the other hand, requires power AND rigidity. Again without seeing your design, an all metal build could certainly have the rigidity needed to hold the tool in position. The steppers may not have the power to make the cuts though. If the steppers aren't strong enough you risk the tool not being pushed into the material hard enough which would result in spinning the tool on the surface, generating heat, and potentially burning the piece if it's allowed to sit for too long. unfortunately the qualities of a machine that make it good for milling, make it not so good for printing and vice versa. Not that a combination machine couldn't be built but you'd likely have to spend more money on beefier components like slides, bearings, drive screws, steppers, etc.