Dont know for sure what that all is. They probably had an EQ, CD changer or sat back there and left the wiring opposed to having to dismantle the rear to get it out properly.
As for King of Clarity, McIntosh is that king
$500 isnt the most amount of money to work with, but its not a bad start.
I would look into a JVC Arsenal CD player. They tend to sound pretty good, decent looking and priced fairly for what you get.
Cadence has a big deal on amps right now, but you have to be a member of this forum to get it:
www.caraudioforum.com
http://www.cadencestore.com/ProductC...&idproduct=257
That can be had for $130! Get on it. 75 wrms x 2 to drive your fronts should suffice well. Thats what Im currently running for power
For subs, its totally dependent on how much bass you desire. I would highly advise a sealed box also.