Mate you've got the right idea with the buying the amp then upgrade gradually.
i would personally (sooo many people go on about 4 speakers, bla bla, but this is me) go for a descent 2ch say around 2x70-120wrms, descent make. I would only run the fronts off the amp & nothing else. I would then buy a mono amp or 2ch (& bridge it) & run a sub.
The problem with rear speakers is it drags the sound behind you. Your ears hear things best that are in front of you.
Also whats the point in spending money on a descent front end, then ruining it with sh!t quality speakers behind you ruining the sound.
Your stereo is only as good as your weakest link!