Yes, it is important to change the coolant, particularly with a BMW with its alloy head. When you do it it’s vital you use the correct coolant as specified in the owner’s manual. Older BMWs, particularly the 3-Series, can be troublesome with high mileage. There are a number of plastic fittings used in the cooling system, including the radiator, and they are known trouble spots. The power steering can also give trouble, the suspension bushes often need replacing, and there are many others. Only buy one if it’s a low mileage car with a known history and a full service record, and only if you’ve got deep pockets.