1) As others have said it's all about what you want. Its' EASY to be low and loud. But to be low and GOOD is another thing all together. Personally I'd take a JL 8W3v3 over a pair of rockford fosgate 12s. But I'm more about SQ than waking the neighbours. (hence my sealed cab JL 10w6) If you want impressive low end, than a pair of 10s or a 12 should be PLENTY in a ported cabinet in a mz2. Anything more than that and your mid-highs wont be able to keep up without SERIOUS augmentation. To be honest, a well designed ported 10" should rock pretty hard. (I'll admit my 10W6 is probably overkill from an output standpoint in this small car, but nothing tops it for SQ) But if all you want is boom well then... If you want a little bit to fill in the lower octaves, you might want to look at a single 8" or 10" ported cab pushing around 250-300W. Remember with wattage ratings that you don't get twice the output from twice the power... you need TEN times the power to double the output. So the difference between 300W and 500W is less than 3dB output. If you want to shake the windows and feel a thump your chest than a GOOD ported 10" in a larger cabinet should do the trick at around the same wattage. If you want to announce your presence before you're seen and you don't care about balance with your mid-highs than a good ported 12 or a pair of 10s will do that nicely with 500W behind them. If you wanna blow out the door seals and entertain people from 6 blocks away, then a pair of 12s with 1000W will probably do the job. Just don't try it with the windows up.
For subwoofers my choice is pretty much gonna be JL all the time. Focal also is up there. Kenwood on a budget. To me, unless you JUST want boom, RF, Sony, Kicker, Clarion, etc.. won't perform.
To genericmoniker's point, that Alpine M-500 is a REALLY capable little amp. You can find them for cheap and they really are quite good. I ran one in my set-up driving an 8W3v3, a 10W3v3 in a high output cabinet and my current 10W6 and it performed quite admirably in every application.
2) Metra and Schosche make adaptor kits for our cars. You'll need a wiring harness kit and an antenna adaptor as well. I'm using metra and the quality is excellent, but i've heard the same for Schosche as well. I think the metra kits run around $60-80 for our cars.
3) I'm a Kenwood man myself. I've had a KIV700 (which I'm actually trying to sell...) that seriously rocked as far as features and SQ and tuning capabilities. That said, I really wanted a double DIN unit for looks and control so I went with another Kenwood deck. I also like Alpine, I just ended up with Kenwood. If you're planning on getting a separate 4ch amp for your mid-highs, you don't have to worry about amplifier power in the deck. Otherwise look for something that will put out 50W or better into 4ohms if you plan on replacing the factory speakers. If you're sticking with the stock speakers and dont plan on replacing them DON"T SPEND A LOT ON A DECK. You simply won't see the SQ increase on OEM speakers. If you're thinking you might upgrade the factory drivers at some point, then it's worht spending as much as you can afford on a deck.