As long as the OP is willing to forgo the steering controls, I'm thinking it's going to be just as easy, maybe easier, to install the factory HK stereo as any aftermarket unit.
Usually, to install a aftermarket unit, it's an adapter to fit the head unit in the same space as the factory unit and a wiring harness adapter.. If the aftermarket unit needs a separate amps, it's no different then the HK unit, since most amps are installed in the trunk space or under a seat..
I feel the only reason to go aftermarket is you usually get a much better unit for the price.
