If the owners have looked after it very carefully and changed the oil regularly then 50k miles is not a huge mileage for an F650GS engine. The engine is fitted with a Nikosil lined cylinder which means that the bore should last a long time. The engine bits that will probably wear are the clutch plates, the valves and its seals, and possibly the water pump seals. Therefore it's important to check the engine runs well, the clutch doesn't slip, the engine does not smoke and there's no water in the engine oil and no dripping from weep hole under the pump. All the other bits on the bike including wheel, steering head and suspension bearings can wear out at any time and are easily replaced and are probably not something to worry much about. Finally, the component which does give problems and is difficult to repair are the rev counter and speedo dials. Check that both work properly and don't jitter because this could mean a faulty stepper motor or a problem with the electronic board.