Problem with mercedes A class 140 Help please?

Hi I have a mercedes A class 140 which is running very poorly and spluttering. I think the lamda sensor may be shot because it is reacting as if the mixture is wrong. I have now discovered that the oil filler pipe is full of milky sludge and there is a slight white smoke and occasional drips of water from the exhaust. also the oil level seems to have risen but the dipstick is not milky. Am i right in assuming that the head gasket has blown and this has ruined the lamda sensor causing it to run terribly bad. The car has only done 52,000 miles. is it possible for a head gasket to go this early and how much to replace?
Thanks in advance for any help