White smoke and a rattle ! Unless the car is dirt cheap and you're prepared to spend plenty of money putting it right, take a/c docs advice and walk away.

White smoke in a diesel is an indication of an incomplete burn of the fuel usually caused by a lack of compression in one or more of the cylinders. It could be a head gasket failiure, valves, pistons/rings or bores worn. If its had injectors as you say, they could have caused mechanical failiure of any of the above previously by over fuelling and now the current owner, after shelling out quite a bit of money without it being fixed, is looking to offload it as it will cost too much to repair.