The factory amp doesn't play well with 4 ohm speakers.
Also if you have the 9 speaker setup, the rears act like 'subs' since the rear doors have sail speakers. The speakers that I've used with the factory amps are either infinity or kickers that were rated for 2 ohm.
If you buy a high watt speaker its going to sound like garbage because there is not enough power running to them.
For example, my intrepid doesn't have the infinity system so I'm running Alpine type S components and infinity reference out back off of my clarion deck (20w RMS) while they're rated up to 100w RMS. There is no bass in the car, hence i have a 800w 4 channel amp to negate this issue.
Your best bet is to either get 2nd hand speakers out a yard car or find 2 ohm speakers. I've never had issues with the factory 6x9s so not sure why yours went (I replaced the stock 3" and 6.5" multiple times).
Contrary to what people say about aftermarket decks, there will be no issue with the factory amp. The deck will not add any power to the speakers since the amp controls this function. There is one trick with the antenna wire, so be careful when wiring up the harness.