Suspension bushes wear quickly on BMWs, its not a problem but rather something to be aware of. Life expectancy is circa 30-40,000miles.
Some cars will suffer from warped brake discs so ensure the car brakes well without judders or strange noises under harsh braking.
Engine sensors can play up, there's about 4 main ones and each costs about £60 ish but they are usually not pin pointed unless they fail but do contribute to poor running, cutting out etc.
Engines & Gearboxes are pretty tough, but BMWs fall into the hands of people who can't afford to service them (not that they are expensive to do so). So ensure the car has been servcied regularly and according to the service schedule (oil service, inspection I and II).