Hi,
a stepper is just a motor, so it will not 'lift' anything...as is. It needs to be connected to some sort of linear motion actuator.
If you want precision and reliability the ballscews are the way to go. On Ebay and other places you can buy C7 grade rolled ballscrews including support bearings for say $100 each for
16mm diameter, 5mm pitch and say 700mm long. C7 screw are +-50um per 300mm and 30 um cyclic....is that accurate enough?
You can get ballscrews in 10mm and 20mm pitch, and they travel very fast but with less thrust. 5mm pitch screw travel slower but have more thrust. With 5mm pitch screws matched to
(low inductance) 23 or 24 size steppers of say 500oz.in you'll move your 50lb easy, and at considerable speed and repeatability.
Craig