If you just want something to replace your nav then I'd say a mobile phone with an aux cable to your OEM system is the best solution.
I installed a Nexus 7 into my M3's dashboard and its fully integrated with the car ; multi function steering controls, DAB ,freeview TV, satnav, internet,etc.
Turns on in 2 seconds when car is unlocked/ignition on and turns off when ignition is off + car locked/10 mins elapses.
I use Timur's rom
bought a DVB-T2 module
USB DAC
4 channel DSP amp
USB hub to plug everything together.
microcontroller to read the car's iBus/Kcan (i forget which..it reads both FYI but one is used for the multi function controls etc etc)
