I just got a Camry a few weeks ago. My old car (Civic) got totaled and I was planning on doing a new sub/amp install in it. I had a buddy of mine at work wire it up for the amp/sub, but I didn't get a chance to use it.
I already have a Pioneer DEH-1300 headunit. It came with the car, but its decent compared to the OEM headunit.
Your head unit is a bit under optioned, but that's ok. It sounds like you are trying to create a basic budget system.
You are correct that the constant power rating (RMS) is the important number to watch. I believe that either of the amplifiers you listed are in the reasonable power range for your subwoofer. The MBQUART is a slightly more reputable company, but looking at the fuses the soundstorm may put out more power. Many companies aren't truthful about their power ratings so it is hard to be sure what an amp can really do.
Class AB amps like the ones you are looking at aren't very power efficient. If the MBQuart says it puts out 400w at 2 ohms (probably at 14.8 volts) it needs to be able to draw at least 28 Amps, but that would be at nearly 100% efficiency. AB amps are closer to around 60% efficient rather than 100%. So if you see a 30 amp fuse rating and it says 400w rms, you know that they are fibbing. The Soundstorm amp has a fuse rating of 50a and it can reasonably make around 400w rms, but in no way can it make the 900w rms it claims. You can also look for "D Class" amplifiers as they are much more efficient on power and won't be as hard on your cars charging system.
Look for a "CEA2006" rating on car audio products, Its a self regulating standard of some of the car audio companies, and it means that their ratings are more honest, or even underated. It's just a rule of thumb though as companies like MBQuart have a reasonable reputation of quality even though they fudge the numbers a little.
Don't worry about replacing your rear speakers, and spend your money on better speakers up front. Unless you have a significant need for rear passengers to hear the music, it is a waste. A component set like these:
You dont say what sort of sound you want etc or whether you have any front speakers. Although focusing on rear speakers used to be how it was done, you may want to consider doing the front speakers first. After all you dont sit with your back to the stereo at home do you?
If you do go that way, you can just pop some front speakers in. Coaxial are considerably easer and a bit cheaper than components. If you're not adding an amp. for these then look for some with decent efficiency (you should see this listed in the specs - 90+ is very efficient) as these will give more on your head unit. You can do without rear speakers. Look for speakers which are a custom fit for your car or buy some simple adaptors to make your life easier.
If you are not amping the front or rear speakers, then your suggested sub and amp. options are probably overkill unless you want a sound dominated by the sub.