Rob-T's recommendation is sound, especially with reference to the longer neck towball that you need for an AlKo stabiliser. However, if the car is already fitted with an AlKo-specific towball, but it has gone rusty, I prefer fine wire wool to remove rust rather than emery paper because it gets into all corners better than emery paper. Only if the surface is badly corroded and rough would I use emery paper to smooth the surface first and then touch up with wire wool.