Hair is so individual, even within the same family, that it may take some trial and error before finding the right hair soap bar that works on your unique hair, in your particular water. If you try one, and aren't happy with it for your hair, it will still be a great soap to use on the body. If you have been using commercial products on your hair, you most likely have a build up of silicone and other "residue" that has made your hair artificially shinier, or slicker looking. Using a hair bar will start to break that down from your hair shaft, and for a time your hair may not look as good until it is all gone. The transition will be worth it when your hair is allowed to be healthy and clean without toxic ingredients.
All commercial shampoos must contain preservatives, but a hair soap bar does not. They kill bacteria and mold. Hair bar soap does not need a preservative, as the PH of the soap is on the high side and the very texture of it helps prevent the growth of nasties in your NO~Poo!