Thats a bummer, but yeah it definitely sounds like the headgasket is shot.
I have a 300SEL as well (1988) with 156K, but thankfully I dont lose any oil or coolant. I dont have any service records with my car, but I am assuming it may have already seen some sort of replacement since I am not leaking or burning any oil.

I just hope it stays that way.
Good luck.