So far everyone has been a great help; I'm going to ask for another piece of assistance. Since I don't have the core rulebook yet, I had intended to pick up the Black Reach starter set. This was with my intention of turning the normal marines into Grey Knights (which now I realize wouldn't work). So my question now is this: Do I buy the Black Reach, which will give me the rulebook, a bunch of marine models that I need, and some orks that I don't, and just build a Marines army with my old figures added in, and then just add Grey Knights into that army....
Or, do I buy the 50.00 rulebook, and just start building the Grey Knights army up with the smaller packs of models? I'm willing to spend either amount of money, really, so that's not the problem.