I would suggest you allocate money for an amp for the front speakers, no matter which you choose. To most rear fill is not that important, so HU power on them would be fine on them. But front speakers, even coaxils with an amp that puts out 30w-50w each will make a world of difference in the sound.
Personally, I wouldn't even change the stock rear speakers unless they are blown. That saving right there is a small amp.