You are right about the Head Gasket, that is probably due on a car with that mileage (not for the first time).
Check the delay between reverse and Drive, I would suggest the transmission is next to go at that mileage. Those are the two majors, next would be suspension, bushings, check the timing cover for leaks.
I would look in the coolant reservoir, if it is brown or green you would be best off to replace the cooling system. Of course the MB recommended coolant is yellow so anything else is doing damage.
Check the power steering/suspension pump for leaks, those are an expensive item to replace.
That is off the top of my head, hopefully something in there helps you.