we dont' hve that model in u.s. but skimming or maching the cylinder head is common. Just to make sure when the head gasket blew out the heat didnt' damage it flatness wise. It is quite comon to do that. alot of the thing of how much can i expect mileage wise is due to afew things. First thing is how you take care of your car, oil/filter changes and if coolant level is low do you still drive it? That ma have happened in this case with teh previous owner. It was low on coolant and he continued to drive. Also if the car is trurbocharged do you constantly hammer on it because it's fun? a car even if it has had something like this fixed or an engine rebuilt for that matter will give you good long service if you take care of it maintainence wise. And no it woldn't have been obvious unless you kept getting a coolant loss and it wasn't checked out.