Rare for an M104 motor to blow a head gasket? Talking to my ex brother in law who is an MB Tech, I get the idea that he does about one a week.
I wouldn't plan on changing the headgasket as preventive maintenance, however. Your motor may go forever without blowing one.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in