Very cool project !
Steppers would be fine. And as you say below, the sound would add to the effect.I'm thinking of making the battle damaged arm moving at the shoulder and elbow . Basically use the mannequin as a mold and make new parts with joints .
I think it would be cool to have some degree of rotational movement in the waist also.
Would you guys use servos or steppers for movement in the arm ?
Steppers have some, but very small, holding ability when powered off. To get them to go back to their previous location when powered on will require an absolute encoder attached to the stepper, and full computer control. There are gearboxes available for steppers, look on Ebay for geared steppers. With a gearbox, there will probably be enough resistance to prevent movement when powered off.When steppers loose power , I believe they also lose there ability to hold, and when they power up again , would they go back to there previous locations ?
Another option would be geared servos or I believe somewhere I've seen geared Stepper motors .
I think steppers sound better robotically, and would be more realistic
Once you get the mechanics worked out, then comes the real fun of getting it all under computer control. There are many options available for this type of project. The degree of sophistication depends on your budget. I would use a stand alone motion controller for this type of project. Small enough to install in the body and no external computer required.
Looking forward to following your progress