The M-Class has always been sold as a 4-door vehicle, unlike some other Mercedes SUVs that have been sold with 2 doors or even as 2 door convertibles in the G-Class.
The first generation of this vehicle was the W163. It was sold from 1998 until 2005. Some interesting features of this SUV are stability control, which detects when slip occurs at one wheel and directs power and specific braking to the other wheels until control is regained.
The build quality on these cars was not great for a luxury vehicle. Owners often complained about this, calling it the Alabama Mercedes to make fun of the infamous build defects that are the trademark of America's unionized car workers. Top Gear host Jeremy Clarkson once poked fun at this car by demonstrating that he could place his finger in the gap between the rear taillight and bumper.
The ML430 was used as the Popemobile at one point, with a bulletproof glass chamber in the rear.
All of these SUVs were sold with either V6 or V8 engines. The transmission is either a 5 speed automatic or a 6 speed manual.
The new M-Class is the W164, which features upgrades in both engines and exterior. Now several diesel V6 engines are offered, and the top V8 is a 6.2 liter monster.
Models of these cars include the ML350, ML500, ML550, ML63 AMG, ML280 CDI, ML320 CDI, and ML420 CDI. The CDI models are diesels. The AMG is a performance model with over 500 horsepower.
These M-Class SUVs can be found used on eBay. There are plenty of listings for barely used vehicles as well as parts.