Have a few 300 Tdi's in the family circle - one has 146,000 mls and head has never been touched and given no problem . Had to replace the head on one with 138000 mls but this was because of overheating - a 'weakness' of the 300 Tdi is the water pump sits high on the engine and a small drop in coolant level can mean no coolant being pumped and a cooked engine .
I have read on another forum about using a Range Rover cap for the expansion tank and this can be used to trigger a buzzer or light should the level drop .This would I feel be of great benefit to the 300 Tdi and hope to rig up one or two of them .
IMHO changing the head gasket just because of the mileage would be unnecessary - if the coolant level is maintained the head will probably never give any trouble . You could change it and the following week a leaky hose could wreck the head .