@stanger_missle Here's the link for the head unit I put in. This one has a 45 watt amp build into the system. It even has capability to hook up a powerless sub to the unit without needing an external amp. Your link didn't work when I clicked on it BTW. Cost about $250. Mind you, if you do opt for one of these powered head units, you will need to run a dedicated power wire to the battery from the unit. I did not read the fine print on that, and had to order a wiring kit separately.
When I looked around the rear doors, I did not see anything where the speakers should be. I can't guarantee buying a set of speakers, wire, and mounting hardware will set you up for an easy install. I don't plan on expanding into the rear of the van with speakers, I haul stuff too often to do that. What I would recommend for better sound is a few things:
Upgraded speakers, and either an amp and a head unit, or the combo of both like we mentioned will help significantly. I am content with my Rockford 3-way speakers and my powered head unit. If you want to stretch it further, I personally feel like wiring your own tweeters in the a-pillars will be an easier job than putting more in the back of the van. Road noise has to be battled with insulation. I have not gotten to that point in the build yet, but as you can see yourself, there is a ton of opportunity to insulate.
Let me know if you have any other questions!