| Unless you are planning to go back to the states any time soon, just buy and enjoy the car. The 320d is a great car.
As far as converting it to US I don't think it would be cost effective. The main differences are in the lights and fenders. Basically, you could end up having to replace half the front end and a good bit of the back. The German emissions are stricter than the US so no worries there, although I believe the US spec BMWs in general are down on power slightly compared to the Europeans.
By the time you leave Germany they will probably be selling the 330d in US and you can upgrade. | 
