After UV Mapping your low poly model theres several thing you can do, from using photoshop or gimp to paint textures on your UV maps. You could bake your hi poly details to your low poly, not sure if blender does this but xnormnal is awesome fore this. It makes very nice maps. Theres also idie tools like substance or things low level price line like z brush. where you can do all your high poly in this and use it to make Low poly versions of your sculpting. Theres tons of solution. look up, xnormal first.