Choosing Amps?

A lot more benefits to components.  Higher quality and steeper crossovers, higher power handling, and ability to place the tweeter on axis with your listening position all improve sound quality in a car audio system dramatically, imo. 

Try a component design that allows you to mount the tweeter in a coaxial position.  It isn't ideal, but allows you to keep the stock look while obtaining many of the benefits of components. 

As for the amplifier.  Spend as much money as possible on the speakers and your speaker install.  Speaker quality and speaker install contributes MUCH MUCH more to the overall quality and character of the sound than the amplifier.  Most midrange amplifiers all sound exactly the same, for all practical purposes, in a car.  Go expensive on the speakers, cheap on the amp, if you are on a limited budget. 

With the power that you need, all can be had in a single amplifier. There is no need in this application for 2 amplifiers. You have one amp to install, this minimizes the chance of ground loops, it also takes less real estate in your trunk. It then saves you a good amount of $ on wiring (use that for the better front speakers). If you buy the right amplifier, it will also be the perfect amount of power for the application. I would short list the Crossfire VR705D and the Eclipse PA5532 amplifiers right now before going off on the dual amp route. Both are rated very conservatively at 50w x 4 rms + 300w x 1 rms (2ohm on the sub), making more power of course as they both use unregulated power supplies. Count on them being around the 75w x 4 and 400w x 1 mark for the most part (this comes from practical hands on experience multiple times over). I will only recommend things that I know will do the job properly and have a highly predicatable outcome in the end.

Do it.  While you'll eventually want to replace the Lanzar, you'll be far happier in the long run if you invest in the best speakers and installation you can afford.

If I were you, I would go ahead and do the front stage first, subwoofer second, and the rear stage last.  Buy a high quality 5 channel amp to start with the front stage speakers, and then add the sub and rear stage next as you get the cash.   Both amps forbidden  recomended are great choices.

That's pretty shallow.  If I were buying a subwoofer right now, it would surely be from Adire's brand new line.  A single 10" Koda or Brahma.

http://www.adireaudio.com/Mobile/KodaSeries.htm   or

http://www.adireaudio.com/Mobile/BrahmaSeries.htm

The Koda is more shallow, I would find some way to squeeze it in.  Retail is ~$220 on the Koda.

First, if you're going for an SQ vehicle you don't need much power.  Second, remember that every doubling of power only get you 3db more spl, max.  Third, most single 10" subs can't really handle that much power, especially shallow mount ones. 

So.  Don't worry about not having much power.  150w for an SQ sub, without eq being used, should be sufficient. 

You have pointed out the two negatives to a five-channel single amp system.  Lack of flexibility and difficulty finding a single large mounting space for it. 

I find a way to get the Koda in your car.  That's my recommendation.  Try this amp: 

http://www.eclipse-web.com/amp/xa5000.html

Find a way to mount it. 

You'd have the base of a killer SQ setup.