Right, templates don't provide BAB or hit points, etc. Generally speaking, they can't be taken as your first class level, you have to take a "normal" class level at 1st level. The best way to take on a high LA template class is to intersperse the levels between regular class levels. In other words, take a level of Rogue, then a level of Unholy Scion, then another level of rogue, then another level of Unholy Scion, etc (using Rogue only as an example, it'll work with whatever class you like).