Don't buy rear speakers for now. You can go for the 8" version of that sub, but the HDS2 has lower power handling. Keep in mind you can buy them directly from the soundqubed.com site as well. Are you limited in trunk space? What kind of a box will you be putting it in?
Btw I have 2 buddies that are running the HDS300 subs. Both didn't want to spend a lot and both are super happy with it. One has the 12" in a box custom built for it...output is insane. Another just wanted to add extra bass, has the 10" in a small ported prefab - does just right, not too much. Those subs are definitely versatile. If you get the HDS, you can always put it in a better box later, throw more power at it, and suddenly you have a hard hitting system.