stepper motor ratings

[usamazahid1]

i am making a robot which will be driven by two wheels(rubber light used in rc cars about 7cm in dia) .my overall weight will not be more than 3.5kg .Which stepper motor with which current,voltage do u recommend .i will use gears with load.there will be a single motor instead of four driven by using chain. and two motors downwards 4 mobility.