it would be infinately cheaper to get a portable sat nav like a tom tom.
you can do it but it is well over £1000 UK money to do it from a bmw dealer. and you are still looking at about £400 to do it with second hand parts. it all depends how deep your pockets are. you can get a tom tom1 for £100 from amazon. a great sat nav unit that is easy to use. if you are from the US im not so sure about the sat nav but im sure you get tom tom there. but over all it is not worth a conversion.