Cadence is a very well known brand, especially around these parts... They've been around a very long time, and make good budget gear, and some upper-tier gear as well. I don't know where you were looking that you didn't find much about them, but they're extremely well known. As for the 7"x10", is that a speaker??? If so, then just simply no... that'd be just silly. If you ARE looking to do some fiberglassing though, you'd be MUCH better off fiberglassing your own kick panels as opposed to those Q-logic ones. From what I've heard, the only benefit of them is that it moves your speakers to the floor instead of the door. The mids and tweets are aimed terrible, the air-space is way off, and the enclosure itself resonates awfully. If you do decide to stick with the Q-logic, at least be sure to line them with deadener.
If you already have a proper component set up front, the dash speakers will probably just screw up the staging and imaging... might be able to make a decent 3-way set-up out of it though, never know.
The amp will be plenty for the subs, you'll be able to turn the gain down a bit //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif I don't think polyfill will be necessary, but you can always try it and see how you like it.
If you have the Pioneer AVIC you definitely don't need the Cadence EQ. And especially not if you have the MS-8. While you could benefit the MS-8, I think the Pioneer will be a fine processor for your needs and goals.
With that Sundown 3500, it will probably take most the current of the Knu 1/0 (I hope it's OFC?), so you will probably need a run of 8 or 4 gauge for the speaker amps.
And lastly, I really hope you have the appropriate electrical to back all of this...