A two channel amp will drive four speakers but you won't have front/rear fade.A pair of speakers in this case could be two coaxials (twteeters built in) or a pair of midbass + tweeters + crossovers.
If you read the guides thoroughly it's all explained in great detail,but you really don't need amped and uprated rear speakers.If you've set yourself a particular budget then put more towards the front end of the car and the sub if you're thinking of getting one.
Either a two channel for the front and a monoblock for the sub,or a powerful four channel for the fronts AND the sub.
Chris.