A ported box is better for SPL ("loudness"); a sealed box is better for SQ (sound quality).
A 10" is better for SQ, though a 12" will extend the lower frequencies better.
2ohms vs. 4ohms : If your amp is stable to 2ohm, you'll get more power out of the amp at 2ohm than 4ohm (GENERALLY twice as much at 2ohm than 4ohms). Some argue that 4ohm will sound better due to better control, but I'm not convinced.
If you want truly good sound, don't worry about the rear speakers. For good imaging, you really want the sound stage in front of you, so any sound from the rear will pull this back a bit. For this reason, many folks (myself included) remove the rear speakers completely. Of course, if you ride with folks in the back seat a lot, you won't want to do this.
Save the money that you'd spend on upgrading the rear speakers and put that towards nice front speakers. If given the option, go for 6.5" coax or components (better), as you'll get better midbass (frequencies above that covered by the sub, but below that covered in the 'midrange') than with 5.25" speakers.
Do you have an amp for the speakers (not just the sub)? If not, then I wouldn't be concerned about speakers "taking a beating", since most head units only put out 22w RMS (the claim of 4x50W is for MAX power -- RMS is limited to ~20w in most cases). If you get a nice set of fronts, you'll want a ~50w RMS per channel amplifier (or however much power given the speaker you choose).
Before anyone argues with me, read the first two words in this post. There are always exceptions, but GENERALLY SPEAKING, the above holds true from my experience.