yeah, I agree with the predator. Its no good getting a lot of bits n pieces if your not gonna get quality components cos all you'll end up doing is complicating the signal path which increases distortion. If you look at it another way: say if you had an awesome sub but a crappy amp, all the sub will do is let you hear the crappy signal from the amp clearly. Stay simple and get good bits, that way you can always add to what you've got, and not suddenly decide that what you have is crap once you get something better. A really good 4 speaker or even 2 speaker config if its set up right will destroy some of the no name budget sub n amp packages.
Actually, if audio is not ur priority you might want to consider some 6x9s and ditch the sub idea. True 6x9s aren't good for clarity, and most audiophiles would stay completely clear of those, but they are a good compromise for base. Think of it this way: the oblong shape stuffs up the clarity, but each one has approx surface area of an 8 inch sub - hence good for bass, not clarity. Would probably be ok if u had nice splits (ive never tried/heard this before though). At least you'll save on weight and space. But to get to the full potential of good 6x9s you'd need to run them off an amp though. The head unit will probably underpower them.
But then again, in audio it all depends on how deep you wanna go into it and how picky your ears are. ie: no point getting $800 splits if you cant hear the difference between $400 splits - some ppl can, some cant. Do what sounds good to you. Hope that helps. btw, when ur testing out spkrs on a demo wall, listen out for differences between spkrs on the same setup, same song cos they will inevitably sound completely different when installed in ur car.