I have fitted a towbar to a 2007 320d M Sport Estate. Local fitters said there wasn't a kit so I searched and contacted towbarsdirect.co.uk and ordered a Westfalia Kit. I had used these before and they are completly invisible. The electrics swing out of the way and the U shaped attachment slots in when you need it. Cost 215 plus fitting.
I could show you a photo but you can't see anything when the bar is off.
Hope that helps