Regarding the woofers I would recommend getting two 12" inch subs in the trunk. Power them through a 400 watt amp. Take out the first aid kits and use them as ports. This is the setup I have and you can hear me coming from a block away.
My friend has a bass tube in the trunk of his benz and you can barely hear it. Another friend has one 12" and it sounds like he has a really good stock system, you wouldn't think he has a sub in the trunk.
If you get 2 subwoofers and put them in the trunk your rear speakers will barely be heard. Therefore I wouldn't waste my money on expensive rear speakers. But if you got the extra money, then you might as well go for it. I would go 12" not 15". 15"'s are loud but unclear. I would go 2 10"'s instead of one 12" just because I've heard both and 2 12"'s sound a lot better.
My subs are Kicker XPL's. The sound is awesome. I would recommend Kickers, Cervin Vega, or Rockford.

Mike