Zbrush has some built in tools to create a low poly mesh that you can then bake the details to with a normal map. Or you can also export it to Maya and do it there.
As for hair, hair is tricky, most of the time you would model it out of planes that use a hair texture. At some point there will probably be a hair option in UE4 that will be similar to the hair tools that many 3D programs have. Depending on what the hairstyle is like there’s some different options for physics.