Ooh that is pretty cool!! Now not being a photographer, I have no idea what to use it for. Looks like it may be a bit much to put my cell phone camera on.

How heavy is the payload gonna be? camera + lenses + whatever. That product says it will lift 10 pounds vertical, do you need that much?

Not sure where the servo goes, for focus maybe?

What are your plans for the actual slide? the easiest way to hack one would be an old printer. but you can buy the slides ready made ($$). Or if you have some tools, you can make 1 without too much trouble. The stepper size will be based on the load to move. theirs is a nema 17 size. I would reccomend a unipolor stepper. there is plenty of debate over which is better, but running at 12Vdc, I find uni better. Bi-polar is great at higher voltage and high speed. but you will have neither here. Also drives are a bit less expensive and easier to build yourself.