My installation was more complicated than yours due to the nasty fibre optics of the CDR23 so I had to remove the amplifier. I have the Pioneer 940BT and replaced the Porsche amplifier with a Alpine 2 channel unit and added rear speakers. Front and rear speakers are connected to the Pioneer and the subs in the doors are connected to the Alpine amp which is in turn connected to the Pioneer by RCA's. Not a difficult job, there are lots of guides around showing where to cut the fascia, run the RCA and amplifier trigger wire.
Head unit selection is very much personal choice. There is no need these days to opt for a built in satnav unit like mine. The Pioneer Appradio units for example use the hardware in your phone, the phone is plugged into the headunit and displayed on screen by iphone or Android apps. I never use CD's, I have a 16gb micro sd card with all my music on but you can also play music off your phone via a lead or bluetooth music streaming.
Yeah, I know the steering wheel is on the wrong side:
