quickest way to custom texture a low poly human model

I need to quickly create a rigged 3d human model. I'm very short on time so am thinking I can just find an existing human model and add my own texture for custom clothing.

My question is, what's the quickest way to achieve this? What is the simplest program (for a mac) that I can use to create custom textures. is it literally a case of opening up photoshop and painting over an existing stretched out image or are there simple programs that will let me paint onto a 3d model?

Also, can anyone suggest a good low poly rigged character I can just drop into unity and use?