The H/K setup from MINI is very expensive to retrofit... more expensive than the less common Digital Power Sound Module I understand - aftermarket is the way to go on this front I recon... Enjoy your car!
I know quite a lot of people on here have used MikeyTheMini from New Mini Stuff for aftermarket stereos and he has a very good reputation on here. Give him a call. I'm sure he'll sort you out with a better system for a smaller cost that retrofitting the H/K system.