Ok, let's clear a few things up.
1)Capacitors are pretty much useless. They are a bandaid, nothing more. If your electrical system is not up to par, you need to upgrade, not patch.
2)RMS spec is not the thing to look at when buying components. It doesn't matter. You need to focus on getting a set that sounds good. Many are out there for $250 or less.
3)Ditch the rear speakers altogether. Having rear speakers will mess up your soundstage. Having a good set of comps up front will sound better and will be more than adequate for people who sit in the back. The most important thing is that it sounds good from where you sit, because you will spend the most time listening to it. You can find really nice comp sets for $250 and less. If you must have rear speakers, just get a pair of good coaxes and power them with the HU. It will sound just fine and won't cost as much or pull as much power.
4)Making sure your doors are well deadened and your speakers are installed properly is just as important as the actual speaker selection. Make sure you include deadening in your budget or you will have waisted a bunch of money
Now, with $900 for everything, this is what I would do.
HU:
Alpine 9886 $270 shipped
Speakers:
Pioneer TS-C720PRS $290 shipped
Amp:
Alpine PDX 2.150 $240 shipped
Deadening:
Raamat BXT $140 per roll shipped (one roll will be more than enough)
Comes out to $940 shipped total. You could easily bring that under $900 depending on where you buy. You could also pick a different HU and comp set to bring the cost down. There are plenty of great, affordable options out there if you take the time to look, especially if you don't mind buying used.
Check the classifieds here for great deals on great equipment. Your dollar will go much further.
I would not buy anything from Kicker except amps. The subs are okay for SPL-oriented installs, but the SQ is really lacking with their equipment