One of the most expensive problems that you can have with any vehicle is to find the head gasket has gone. Often you may not even have any warning but when it goes it can cause so much damage that the car will become un-driveable and will require a new engine to fix it.
You can of course buy a reconditioned engine to fit in it but you will have to make sure that you buy one with some sort of warranty as you could find yourself in the same position a few weeks or months down the line.
The labour costs of fitting a new engine can be more than the cost of the engine itself and therefore you have to weigh up if the vehicle is worth repairing or if you are better off to scrap it. If you are considering repairing it then you will need to take it to a garage or have a experienced mechanic fit the new engine as if any part is not fitted correctly you may end up damaging it straight away.