Neil - I can believe that for the traditional rite of exorcism the st benedict medal is required. However, that does not mean a shadow is cast on crucifixes that are not, not that you are saying that. So, I don't think the devil mocking his potential pick is what mcollier needs to be hearing(not that that is what you were doing). But, I don't think this is what he needs to be considering. I will agree that it is not recommended to make a crucifix without the INRI or corpus(3D or not). I don't think nails are essential, although I would simply get an awl and poke a hole where the hands and feet are.
I recommend relief carving because I have enough experience with carving. And, carving fine details in wood is very difficult, delicate, and time consuming. We are accustomed to hardwood crucifixes, but that requires high quality specialty chisels which the OP may not be interested yet in investing in. So, if he wants to do something 3d, the ideal would be to use wittling wood, which is a very soft wood. So, OP consider using a very soft wittling wood. Make the corpus and the cross of the same material if you are going to carve them separately, that will probably look best when brought together.