I can see three means of making a hermetic prosthetic. Each with different costs, advantages, and disadvantages. The first is a mentum spell with rego and form requisites. This is one of the 2 all in one packages I can see working. Now I considered "reading surface thoughts" (15) but you aren't really thinking "move my hand, move this digit, etc." hand moment is more of an impulse, a state of thought, you can do it completely autonomous of your thoughts. So base 4.
(The prosthetic limb) Cooler name pending
InMe (ReTe) 24
T: Ind D:Conc R: Touch
Identifies the desired movement of the limb and moves it accordingly by moving the prosthetic accordingly.
Base 4 (+1 conc, +1 Touch, +1 Requisites, +4 constant, +5 maintains concentration)
I choose concentration as the device has to constantly change form with impulse. The fun of this one is you could have your hand running around a room like cousin It. The downside is you need to add another spell to let you feel anything. I guess you could add an additional Imaginum req to add that on.
This may seem a bit low, but I choose this method because your mileage will very with different forms if you make the base effect the Re(form). I have considered adding a +1 to do to complexity, and a +2 for a hand, given that it is far more precise. But first lets look at a Terem hand in the other all in one package and why I don't believe it works.
The Metal Leg
ReTe (InMe) 29
T: ind D: Conc R: Per
Causes a metal limb to move in accordance with the magi's desires.
(Base 3, +2 Metal, +1 Conc, +1 req., +4 constant, +5 maintains concentration)
Now the level lends credence to adding at least a +1 to the first effect. But since the device must effect itself, then we need to effect another target. But like A terem spell cannot effect you. So I'm inclined to throw proposal 2 out the window.
Now we have the multistage situation.
(1) Sense the motives of the (limb)
T: Ind D:Conc R: Touch
Senses the desired motion of the (limb)
Base 3, +1 conc, +1 touch, +5 Maintain, +4 constant
(2) Movement of the lost (limb)
ReTe 22 (27)
T: Ind D: Conc R:Per
Moves the (limb) according to the desires of Sense the motives of the (limb).
Base 3, +2 Metal, +1 conc, +3 Link Trigger, +5 Maintain, +4 constant, (for hand only: +1 precision)
This seems a bit shaky to me, but doable. Again, you need a ReIm spell to feel anything. The downside is the warping accumulation from constant effects in both case 1 and 3.