alright here's something good for you :

you have 2 choices
1st if you go with a 5 channel amp, well you power up everything (but do you really need to? I think that if you only want a better sound in your ride and your not doing this for db drag, then this is not your best choice.)

So now you have to decide if your coax. speakers relly need to be amplified.. because for sure you will need to power your subwoofer and the amp that power a sub. is a mono or a dual channel block. I'd recommend you to go with a 250-300 W RMS amp (mono amp) and to plug a single 12" subwoofer..

For example I would use the GM-D510M (pioneer mono channel amp.) at 4 ohm to use with an Alpine Type R subwoofer (12") or a Pheonix gold Octane ZR sub. (12"too)

so your best bet is to add power to your sub. without powering your coax. + tweeters with an amp because of the tweeters, you don't relly need that much power ....