Editing the model always comes before skinning it, that goes for both making one from scratch or editing an existing one.
Making a model with the only changes being stuff like color variations in the hair or skin color makes it what you would call a "skin model." Typically if you want to make small changes like that, you don't need to edit a model, just edit the skin.
I don't get your last question. Do you mean build one from scratch? -VGsatomi