To figure out what motor to use, I'd suggest using this spreadsheet as a rough guide:
http://www.chiefdelphi.com/media/papers/2059
Go to the "linear mechanism" tab. The two #'s 1885 set requirements for was the "Time to move travel distance under load" (< 3 seconds) and "Max loaded current draw" * 12V < half of the peak power rating of the motor. Miraculously, we found 2 perfect matches of motor/gearbox/sprocket combos!
Note that the output current cannot constantly exceed the fuse for that particular motor, for whatever your load is. Actually, if it exceeds a certain amperage consistently, you will burn out the motor. Also note that with a 2-stage elevator, your load is either ~0.9(2*lifted weight) or 1* the weight but you'll have to move 2 times a single stage's travel distance, depending on how you hook it up.
We're lifting ~28lbs including the ball over 3 stages, so we're accounting for 90 lbs and have a slightly larger sprocket ready if we decide it's doing well enough to add some extra speed.