Since you like the bass of rap and the undertones of rock, two 8s and one 12 should sound decent. One thing I would do, run the different size subs with different amps so you can tune them to your style. As for the headunit, you will need a total of three unless you don't want to use the 6x9 location in the back deck. Most headunits are set up as front, rear, and subwoofer. If they only have two preamps then the controls for the sub is dropped. If they have one preamp then it's generally just the subs, but you really can't find those much anymore.
If you are concerned about price, this is what I would do:
Start with your head unit as that will give you the most improvement of sound quality over stock plus give you more features that you don't currently have. A good one to look at is the Kenwood KDC-MP4028. It has a really good signal-to-noise level and great fm sensitivity (get in those hard to reach radio stations). Can do XM or Sirius if you want satallite. Have a look at
this comparison. Now, $179 at crutchfield isn't bad because you get the install kit and wire harnesses for free. I'd say the kit and harnesses can be found for as low as $20 if you look hard enough, generally you'll spend about $40 on all the kits new (wire, antenna, dash mount). I've seen the Kenwood unit for as low as $100. I'm not pushing Kenwood here, I honestly don't like their lower end equipment but I'm just trying to help you find what you need for your setup done cheap.
Your second step should be to look for a really good dual voice coil 12" sub. This will be your primary bass driver so I would recommend spending a bit more on it than your original plan. Forget about the 8s for now, that should come later down the line. A good DVC 12" capable of handling 300W RMS will more or less do what you need, compare the differences in the subs you are looking at by the XMax values. When you get the 8s, look at SVC 8s at 4ohm and wire them up for a 2ohm load. Whatever you do, make sure the 12 and 8s are not in a common chamber box, one of the two sizes will be destroyed by the others excursion and back pressure.
