Rattles aren't normal on the 250, at least not the pregens I'm familiar with. The only time mine rattled was when something was improperly loose. Main issues to look out for stem from neglect and improper storage. Carbs get gummy, plugs need changing, tires dryrot, chain ignored, sludge in the tank, that sort of thing. Before starting, make sure it has fresh gas and has a good oil level. Check for binding in the throttle cable and for fraying at the top end of the clutch cable. Damage from drops can be fairly expensive to repair as undamaged fairings are hideously expensive and have been for a while. Look inside the tank for rust. If it's been stored outside pull the seat and check for wasp nests.
