A Pioneer 6 disk changer is an awesome package for a 2004 Edge.
You may MIGHT consider swapping out the speakers for Alpine, but I wouldn't mess with what you have in reality. Adding an AUX option would be money well spent.
Aftermarket stuff can be real crap is you're not careful. Replacing the door and rear speakers is as far as I would go with a Pioneer 6-disc system. Now, you may want to add an AUX connection to the setup along with say, Alpine speakers.
But to put more than $300 in materials cost into your system would be wasting money. Get an AUX addition, and PERHAPS, better speakers, and pocket the rest.