Door speakers can be a pain to add, you will need to replace the interior of the door and make a speaker baffle if you install anything bigger than 5.25" But they make a big difference.
The rear speakers will not be hard to do. I would recommend using PVC to make a speaker box, but I have also seen some very cool custom enclosures made out of fiberglass and wood. If you only want speakers in this box, and not storage space you could try and mount a thin sub there. That would make more sense than mounting one in the trunk.
