You don't really need an amp for the components, remember, the amplifier is the most expensive item in your system, and you'll definitely need one for the sub. The head unit should be enough to push the fronts and stock rears pretty loud without distortion, so go for an amp for the sub instead.
Definitely go for the 10" over the 8", you don't really lose much punch and you gain lots of deep bass. I'd say go for an Infinity Perfect, but only because I'm a little biased (I have a 12" Perfect in my 240.)
As for wattage, you wanna match as closely as you can the RMS of both the sub and the amp, but if you can't, go higher with the amp. Subs usually get blown with not enough power, and hardly ever with too much power.
That'll get you started at least.
