artie thanks for the link; I too have been bite by the G wagon bug and have been researching; I'm in Jersey close to philly and they are popular here; they problem I found that maintenance can be sporadic with some of the owners of these new (either can't be bothered or don't care because they don't plan on long term keep) if yu find an actual G you can afford do research and try to find maintenance records. The biggest problem is that parts are very expensive; let alone the guy who will do the labor for you at the Merc dealership; unless you can find a mechanic with the software. Here is the biggest problem; most companies now a days are proprietary with there software; so you need someone who has access to this or pay at dealership
I've got a couple of classic cars including a 1984 Mercedes 280SL. In my experience, the repairs on the engine and transmission of the G-Wagos of the 80's are just as expensive and require the same parts as just about any mercedes from that era. The 280GD has the M110 engine and not many 280 cars were sold in the US so that may be some what of an issue. Plus the 280 is a 6 cyl, but has dual overhead cams, so valve work is more expensive. that engine is amazing though. I've got it in our 280SL and it flys.
The diesels are very easy to get parts for because so many diesels were sold during that period. Anyone that can work on a 300D can fix the engine in a 300GD. Likewise the transmissions are the same, except they vent from the top so you can drive them in deep water. Total transmission rebuild will set you back about $1700, exact same price of just about any other Mercedes from the 80's.
Body parts are also easy to get as are most interior parts. Honestly, I'm paying less to maintain my G-Wagon than my 280SL and it is way more fun to drive. Just get one WITHOUT RUST! Body repair on any car is costly.