Many different senarios are described as a "Blown headgasket". The gasket participates in the sealing of oil passages, water passages, and the compression of the combustion chamber.
Although the repair can be the same, the need for repairs differ considerably. The worse problem is one of combustion sealing. Losses here cause errosion of the sealed materials. This condition is terminal - has to be repaired. this condition is changed further by the introduction of coolant to the cylinder. If this is the case not only does the condition require repair but it requires it immediately as leaving the engine sit will cause serious corrosion of piston and rings to the cylinders.
The condition of an external leak is only important from the stand-point of being able to maintain coolant level and antifreeze mixture for its purposes. If the leak is external and you are maintaining coolant levels then there is no immediate need. The problem with this is that it may at some point become much worse and or spread to an internal leak causing over-heating. Since your car has a coolant level gauge you have less risk here.
It worries me more to see the car set in this case as you probably can not be sure whether you also have a small internal leak.
Bosch Master, ASE Master, L1
33 years MB technician