One reason you might want to go through a headunit is you have easier control over the sound. You also still have sound if the carpc dies or while its being worked on.
I'm not sure why you need two 2 channel amps for F/R speakers. Why not just one four channel amp?
As for wattage, higher wattage = cleaner sound (also louder because cleaner signal at higher power).
When choosing an amp, its RMS wattage should be in the upper third of the speakers power range.
So if your speakers can handle 50W RMS, go for an amp that can deliver 50W/ch. Any more and you risk blowing out the speakers.
YES, get crossovers. They are well worth it. Even just a capacitor is better than nothing. High pass for passing higher Hz to regular speakers, and low pass for passing low Hz to subs.