the guy at best buy was right. an aftermarket will give you anywhere between 5-13wrms before clipping. Clipping is the amp cuts off the last bit of the signal to allow it to produce more power. This is bad for both the amp and the speaker.

You will get better sound from a properly set-up pair of mid-line component speakers than you will with even higher end coax, so I woudl say to look for components. Amping them isnt a much, but at the same time you wont get pull potential from your speakers running them off the head unit. I would personally do a 2ch amp and run your front speakers off the amp and leave your rear speakers on the headunit.
Do you plan on running a sub? If so, forget the 2ch and get a 4ch. This way you can run the 2 front channels to your components, and rear channels to a sub.

I'm not familiar with your car, but I would go with someone that actually looked at the cars opinion over a computer.