For your 'Z' axis you can save your self money and time by using a 'linear stepper'. It will combine your motor, ball screw and linear rails all in one unit. all you need to do is add an 'eye' for a wire guide and your good to go.
Permanent Magnet Captive
Stepper Motor Linear Actuators
Some of the surplus electronics places carry them too, like Allelectronics; All Electronics | Electronic and Electro-Mechanical Parts and Supplies at Discount Prices
The last ones I bought were from Ebay.