What constrains quality on a 3d print?

[printbus]

All the things you mention can affect print quality.

FOLLOWUP COMMENT: The resolution of the stepper motors themselves is pretty standard (200 full steps per revolution). What matters is the amount of microstepping applied (typically ranging from 1/2 to 1/16), and the nature of any gearing between rotation of the motor and linear movement of an axis.

If you're sold on a Prusa i3, I'd suggest taking a look at the i3v series printer kits from MakerFarm - they are a variation of the Prusa i3 that utilize a combination of a veneered MDF frame and aluminum v-rails, with delrin wheels that ride in the v-rail channels for all movements. This combination leads to a rigid frame with very smooth movements.

The MakerFarm subforum here contains a lot of user input on build help, printer mods, and even implementing automatic bed leveling/compensation.