These engines are as durable as the average family car, >100K is common, so if you wouldn't have any issues buying a car with 18K on it, there's no reason to shy away from the bike.
The only thing to really look out for is some 2001-2002 B12's use oil, so if you're looking at one in this vintage, you'll want to be sure it's not one of the 10-20% that use oil. (Unless you're plans call for breaking into the motor for mods and you're going to change pistons, in which case it doesn't matter.) More details on the oil issue are in the the 600/1200 FAQ section.