One issue with retrofitting a system that wasn't originally an option for that model of car, is that chances are that the fitters won't be able to SCN code it, which means that the system will not show the car's Datacard and it will not be possible to generate a PIN code for the maps update. The way around it would be to update the maps while the system is still in the donor car, or buying a dodgy PIN code off eBay.
Apart from the above, it might not actually fit in terms of MOST, wiring loom, integration with the instrument cluster and steering wheel, etc.