I actually put 50 or 60,000 miles on a 2002 G500 and I must say it wasn't as bad as everyone says. I had a couple electrical quirks that were easily addresed with bad ground wires. Power windows are a problem on all G wagons. Finally, the transfer case motor plastic teeth are easy to fix and not expensive.
What I think is very important to watch and be careful of is: what is under the green? How much rust? how many non-factory gadgets and gizmos are in it? These aftermarket "upgrades" (stereos, lights, etc.) cause the most problems in my experience.