I just want to buy some stuff.......
Buy once and for the right reason or you will end up buying lots of stuff.

I will assume that you are retrofitting an existing machine.
How the stepper is wired determines it's operating characteristics. Torque, operating speed, amps / voltage required
etc. The stepper is sized for intended use and you need a desirable torque range at some operating speed for whatever you are going to do. The system ie; power supply, drive and motor act as system, so, before you just buy
you need to understand how that is interelated.
You may want to consider using servo's instead of steppers. They require a different type of drive.
Suggest you consider modifying the motor mounting instead of picking a motor that fits in a particular housing.
So define what you are going to do with the machine, see what type , size, motors are required for anticipated movements, provide a correct drive for the motorof your choice and make sure the power suply is adequate.
Thus go back to the Gecko site and "STUDY" all the info there and know before you go.
BTW, 180 Ncm not "180 Nmc" is likely the holding torque rating in metric units.
1 newton centimeter = 1.4161193228 ounce inch
RICH