If you want to go the motorized route, a good way would be to use a very small motor, say a pager motor, but instead of using a servo which will give you too much torque and will be far too slow, use some pulleys. These can be found from old tape decks, or you can purchase them online from places like ebay or technobots.
With a pulley driving a threaded shaft, you don't require limit switches, when it hits the end stops, the belt will just slip. A further enchancement would be to use a current sensing chip. These will stop the motor when it pulls above a certain preset current, and prevent it from damage.
If you want an example of this in action, open the draw of the DVD/CDROM drive on your metal mickey, when it's half way open, stall it with your hand, and observe what happens. For a short moment it will continue to drive, but then will stop, after a short pause it will retract back. It's an inexpensive and simple way of doing away with the need for feedback and/or limit switches. It only works for low torque applications, but rasing and lowering scopes is in that category.
I've looked at Engel's methods. The system on the Typhoon in particular is a source of amusement to me, as they have taken a sledgehammer to crack a walnut.
Personally I would go for the hydraulic master/slave arrangement, as it's simple, offers the bets control, and is dirt cheap. Plus you have stacks of room inside your dive module(s).