Your 12.1 will work just fine. In fact, TC Sounds was selling off their extras for $75 each since SVS took their driver manufacturing in-house -
link.
I didn't know that Soundstream made head units. They are known for their Reference series amps since they are still getting a premium price on Ebay. I've seen a little discussion of their current subs and amps on the car audio forums but there are definitely better buys out there.
I'm still recommending a HU from Crutchfield. My favorite is Pioneer. The one in my Maxima is 13 years old and still working just fine. I may upgrade to get one that has a USB input so I can connect a 20GB drive to it.
If the ED components are too much, Cadence has some overstock in their online store for under $100. They were $250 when they came out a couple of years ago. Very good reviews. Mount the mid in the stock location and the tweeter in a spot on the door a little above the mid. Just use a hole saw on your drill to make a hole the appropriate size. The trim rings on the tweeter make it a nice, neat install. Or you can go with a nice coaxial from someone like CDT. Any way you go, the doors are extremely easy to work on. They are usually held on with 2 or 3 screws and some plastic push pins. Your local Autozone will have a panel puller for $10 that will make the removal very easy.
Like I said earlier, use the facory rear speakers and power them with the HU or don't power them at all. You want your music to come from the front. I don't use rear speakers at all.
Subs - use the recommended small, sealed box for your 12.1's. The ED amp will push about 450 watts to each. You will have more than enough bass.
Amps - ED amps. Quality brand that has a great warranty. You can also get a good install kit from them as well. If you run into trouble, the owner of the company as well as their tech guys frequent their forum or you can call for help. They stand behind their product.
DIY car installs are not difficult. Once you get over your fear of popping off a panel, it's EASY. If you happen to break a push pin, you can buy a replacement bag of them from Autozone or your local Chevy dealer for a few dollars. Run your power wires down one side of the car and your signal wires down the other side. Door sills are easy to remove and that gives you access to the edge of the carpet to run wires under it. I've seen the trunk of a regular Impala and yours should be at least as large. Your sub box should fit without a problem. There are dozens of pre-fab ones one Ebay. ED will build a custom one for you. Or you can use one of their many blueprints and try your hand at building.
Check out the prices of the equipment I listed. See how much you will save with a DIY install. Break it down into sub-projects and tackle each one.
-Robert