my headgasket(s) have gone on mine. but its not mixing oil and water.
Its leaking pressure from the cylinders in to the coolant system.
But if i turn the boost down and drive like a granny its ok. but as soon as you start to give it some, the water start bubbling over.
Head gaskets can be had from anywhere from 80 pound for the pair upwards depending on what you want. It is possible to do them with the engine in, but most remove it so its a costly job.......
I'm going to do mine myself, when i get space on my drive ( and fixed the head gasket on my galaxy!) plenty of step by step guides how to remove the engine and do the head gaskets on s/net.
So to sum up!
If you do it your self it will be about £100 for the gaskets, £50 to get the heads skimmed, £40 for a cambelt,plus usual stuff like oil and filter etc, and 2 days off the road.
If you do put the engine, do the clutch (i just changed the drive plate last time for about £60) at the same time cos thats an engine out job as well..........