Morning Rob
If you are buying from a Dealer get them to fix it BEFORE you buy. If you are buying Private walk away as there are no "cheap fixes" with a BMW just ask yourself why the current owner hasn't fixed it? Don't listen to rubbish like "it doesn't affect how it drives? Or the parts are easy to get on Ebay ?
It could be a sensor fault, it could be a control unit fault, it could be a steering rack fault so unless you have the skill knowledge and equipment to diagnose and repair it walk or it will empty your wallet faster than you think is possible.
Dave