Hi Amer
Clearly head gasket. Don't spend or waste time on any other part. Even though plain water is bad and you will get the rad rusted in no time and thus it should be flushed and proper coolant put in.
Someome else might give better suggestion but if you really want to keep the car, check egr valve and see if it is clean. Not sure how anyone can verify dpf as well but my theory is that blockage of these cause diesel engines to die or develop head gasket issue.
Now you can be lucky and it can be expension valve but on japanese cars, a faild expansion valve generally tends to remain open.
Diesel are the most dangerous cars in the world. If you have kids, think of their future. Next car should not be a diesel. Many people here will object but watch a documantery called Dirty Money.