As a professional henna artist, yes I do recommend you try out a henna "tattoo" to be sure of the permanent tattoo you might want or get it even designed by a henna artist. This way you can always adjust it as needed.
We usually don't call them tattoos though either, but use the term body art. (Henna Body Art).
It will last from 2-3 weeks. It looks nice for at least one week and then it fades.
Never ever get black henna. It is henna with harmful chemicals added to make the natural henna turn black. Henna is a natural plant that will only dye reddish tones.