Unleaded fuel won't hurt your car. All of the Mercedes gas engines have hardened valve seats. The only change M-B made when they produced engines for use with unleaded fuel was the addition of 2% tungsten to the seat's alloy. I've been running my 250SE, 3.5 and 6.3 on unleaded for 10 years with no problems. The original valve seat material seems to be just fine.
I would use the 97 octane variety when available. My stuff runs okay on 94 octane.
ASE Master Tech
Mercedes Tech 19 Years