The G-Class has been based on the W460, W461, and W463 platforms. The first was a versatile vehicle sold as a 2 door or 4 door SUV as well as a 2 door convertible.
The W461 was first sold in 1991, and looked far too much like a VW Thing, in my opinion. It was sold with the same body types as the original, and was not replaced until 2001.
The W463 has been sold since 1990, as is the most luxurious of the G-Class. It comes with a V6 standard, but can be upgraded to a V8 or even a supercharged V8. That latter option is a 469 horsepower beast. It's only sold as a 4 door SUV, and is very popular with the wealthy and dignitaries, especially in the oil-rich Middle East. These vehicles are not sold by Mercedes Benz in the United States, but they are commonly imported. They sell for around $135,000.
These G-Class SUVs are obviously quite rare, but you can find them, even used, and even in the United States. The first place to check is eBay. If they wealthy original owners aren't selling directly through eBay, chances are one of their handlers or business partners is doing it for them. There are usually multiple listings for these SUVs, and for a lot cheaper than they are new.