I really dont think its worth it to build a system around deck power. I highly doubt the deck will be able to keep up with the bass and youll probably end up blowing something out. I dont know why a 5 channel would be used in db drag. Using an amp to power all the interior speakers and a sub is good if you have the money but it is harder to find good 5 channels. It would probably be easier to buy a 4 channel and a 2 or 1 channel amp for just the sub.
Personally I would run a good 4 channel on the fronts and bridged to the sub then power the rears by the deck. If you want more power then just buy a 2 channel later. Id only figure on getting half of that 54 wattsx 4 of the head unit in clean power, maybe if that.