If you plan on powering your stock speakers on the head unit, you won't go far. Expect that your back will overpower your front.

Never a fan of coaxials. Get separate components -- midbass at the doors and use 3M double tape to mount the tweeters on your dash. Easiest way on how to "position" them is 30 degrees off-axis -- in normal language this means your left tweet should be aimed to your passenger and your right tweeter to the driver.

I'll also throw my vote with the JBL GT-5. If you think it isn't enough, get two. And no, don't get the pre-fabricated GT5-1204BR version, the bump in price is ridiculous. Just get the GT5-12 driver and ask your installer to fabricate a box on it. Is space a premium and you simply want to enhance the bass? If then so, 1 cu.ft sealed would do it.

Take markgy's advice. 4-channel amplifier -- 2 channels for the front speakers, and 2 channels bridged for the subwoofer. This is usually your typical "step 1" setup for most people.