I've never installed a double-din unit into my P5, but I would look into how they're secured and stabilized into the dash.
As for a basic sound setup, I think you're headed in the right direction for the most part. I myself have a Pioneer head unit, a single Kenwood 4-channel amp powering my front Polk Audio SR components and a JL Audio W3 sub. This is a basic setup that I'm confident will be good for the sort of application you're looking for, and there would be no need for a capacitor because you're not installing a high-power system. I don't have a capacitor; my amp is pushing 120 watts to the each of the front components and 300 watts to my subwoofer, which is a very good amount of power for what I want and gives me no issues.
I'm going to kindly disagree with u812many73 regarding rear speakers. The front speakers are the ones you want to focus on. For many car audio enthusiasts, the rear speakers are so unimportant that many don't have rear speakers at all. Some would even argue that
having rear speakers will actually throw off the balance of the system as a whole because they'll interfere with the front sound stage. However, I will say that a lot of people like having rear speakers because they like having a surround-sound type of experience, and certainly having rear speakers will help with that sort of thing. Personally, I have no rear speakers in my P5 and I'm completely fine with it.
If you'd like to have rear speakers, either because you like the surround-sound experience or you often carry passengers in the back, you can certainly keep the stock ones. In this case, however, I'll agree with u812many73 and say that the stock speakers are mediocre. One alternative is to buy an inexpensive pair of speakers, perhaps an Infinity Reference set, and just run them off of head unit power. Either way, since you're amping the front speakers and hearing the music predominantly through them, the kind of rear speakers you have (if at all) will play a pretty small role in your overall experience. The rear speakers are located in the bottom of the doors, so at most you'll feel their presence more than anything.
As for the subs, two might be a bit of overkill, especially from the impression I'm getting of what you're looking for. If you're looking for a hard-hitting system that will make people notice you down the street, then multiple subs will fit the bill. If you're looking for sound quality, I'd advise going with either a single 10" or 12" subwoofer. You'll still get a lot of bass, no doubt about that, but it will sound a lot more musical than going with two. Besides, a P5 hatch is pretty much open to the rest of the car, separated only by that cloth-covered cardboard hatch cover. A single sub will make its presence known.
Now before I make any recommendations, I'm wondering what sort of Infinity Kappa speakers you were interested in. Were you going with coaxial speakers or components? And were you sticking with the 5x7/6x8 size for the front, or did you want to go with round ones? Component speakers are more often preferred, but that would also mean more work to install the tweeters. Coaxial speakers are a lot cheaper, a lot easier to install, and can sound great, though the sound stage is lower because the sound is coming from the bottom of the doors. Components are more expensive and are generally better in sound quality. With components, you can install the tweeters higher up (mine are in the sail panels), thereby raising the sound stage. I've installed both coaxials and components in friends' cars and both can be satisfying, though if you can afford to and are willing to take the extra time to install them, try to go for components.
As for oval (5x7/6x8) or round speakers, the debate rages on. In my experiences, the difference is marginal to the point of being inaudible. Since you're not competing in sound competitions and you're looking for a basic, nothing-crazy type of setup, you can make the install easier by going with the 5x7/6x8 size.
Regardless if you're going with coaxial or components, a very nice amp for the Kappa speakers is the Pioneer GM-D9500F. This is a just-released 2010 model from Pioneer that incorporates the newer, more efficient Class D digital amplifier technology. What this basically means is that you can have an amp that pushes a lot of power more efficiently and emitting less heat, while also being very small. This Pioneer amp is compact, at about 9"x8"x2.25". Plus Sonic Electronix has it in-stock at a really great price. Digital Class D amps tend to be expensive, so this is a great deal. As an example, Alpine has a PDX-4.100 that has similar specs at over double the price.
http://www.sonicelectronix.com/item_22382_Pioneer+GM-D9500F.html
And for product page with images:
http://www.pioneerelectronics.com/P...eo/Amplifiers/GMDigitalSeries/GM-D9500F?tab=B
The Kappa speakers run at 2 ohms, which means that it will draw 100 watts from the amp, which is what the speakers are rated for. Bridging two channels will give you 200 watts for the subwoofer, which is substantial power for a single budget-priced sub, especially a 10" one. My sub experience is limited to JL Audio, Polk, and Alpine, and any of those that are rated for 200-350 watts RMS will be good. I've also heard good things about Infinity subs, though I've never had one or installed one personally. The Infinity Reference subs would be a good way to go, in that case. Matching RMS power ratings is recommended, but there's a LOT of leeway to work with. u812many73 mentioned a good rule of thumb, working within 80% of the recommended RMS range, which means that you would look into subs that are rated for up to 250 watts RMS. However, I have a friend who currently has an amp that pushes 200 watts RMS bridged to a JL Audio W3 10" sub that can handle up to 500 watts RMS, and it still sounds good. Granted, the JL Audio sub is a high-quality sub, but I give you that as an example that RMS power matching isn't absolutely necessary.
A list of decent subs to look into, based on power ratings and what's in stock at Sonic Electronix:
10"
http://www.sonicelectronix.com/item_18573_Infinity+Reference+1060W.html
http://www.sonicelectronix.com/item_19638_JL+Audio+10W0v2-4.html
http://www.sonicelectronix.com/item_8758_Rockford+Fosgate+Punch+P1S410.html
http://www.sonicelectronix.com/item_20057_Pioneer+TS-W253R.html
http://www.sonicelectronix.com/item_11797_Alpine+SWE-1043.html
http://www.sonicelectronix.com/item_18188_Polk+Audio+db1040.html
12"
http://www.sonicelectronix.com/item_18622_JL+Audio+12W0v2-4.html
http://www.sonicelectronix.com/item_11841_Alpine+SWE-1243.html
http://www.sonicelectronix.com/item_20058_Pioneer+TS-W303R.html
Most of these with a matching sealed enclosure will cost you less than going with the Kicker subs you mentioned and the added cost of a dual sub enclosure.
Sorry for the long post, but I think it's good to be thorough. Let me know how it goes.
