The Mercedes-Benz M-Class has been selling faster than expected since the company's new factory in Alabama came on line in late 1997. Perhaps that's because the M-Class is the Mercedes of sport-utility vehicles. More likely, it's because the M-Class delivers an excellent balance of the rugged looks, room, security and utility that SUV buyers seek. There are 4X4 trucks with more ground clearance and better rugged-terrain tires than the M-Class. Yet on streets and highways, where most SUV owners spend most of their driving time, those 4x4s behave like trucks. There are also sport-utilities -- notably the Lexus RS 300 -- that are even more car-like than the M-Class on pavement. But the RX 300, with its sedan-derived mechanical components, has limited off-road capability. Part of the SUV's appeal is the liberating thought that one could leave the highway and head into the bush without a moment's notice, whether or not one actually does so. The M-Class has been successful because it's good in just about every role an SUV is asked to perform. For 1999, alongside the original V6-powered ML320, Mercedes has introduced an ML430 with a 268-horsepower, overhead-cam V8. The ML430 adds more luxury appointments and Mercedes' sophisticated electronic anti-skid system as standard equipment. Better still, M-Class supply may finally be catching up with demand. That could leave more room to dicker at the dealership.