It sounds like you should be looking at Shake.
Motion is more for 2-D graphics, titles, and other eye-catching graphics. Motion’s main point is that anything it does it can do a lot FASTER and EASIER than After Effects can. They never claim it to be a replacement for AE, though, because it can’t do everything AE does. What it can do, it does better, but it can’t do everything.
For any kind of compositing, though, Shake is the place to be. You should probably look into taking a class somewhere first, though. I don’t know Shake at all, but from what I understand it has a much steeper learning curve than the other programs. It might be wise to learn a little bit about using it before you spend any money on it.