The 2002+ are great trucks. 02 and 03 years got a bad rep initially while MB worked out some electrical issues but those should be well sorted by now assuming the maintenance is in good order — you definitely want something with a good maintenance history.
The motor/trans combo in the 1998 to 2006 G500 has legendary reliability.
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.
Yeah as nbutze points out the green color shown is not the shade of green that this truck left the factory wearing. You can see the original green metallic in the door jambs and around the door interior paneling. Cheap paint job
I owned a 2007 G500 for 10 years (bought new} and I loved it. The only problem in 30,000 miles was a window regulator
failure ($1000) repair at the dealer. The problem with Gwagens is the extremely complex systems (at least 12 separate computer modules) that
can and will fail. Each module is many thousands of $ to replace. As long as you are prepared to spend that much for repairs, go ahead.
You can also find much better examples for the price. That one you linked was a 2002 G500 with ~120k miles going for $29k. Should be closer to the $25k range if it was in good condition. There are relatively good deals to be had for G wagons now. Just keep looking.
Main rule when looking at G-wagons is look for the motor/transmission combo you want, then look for the newest year/least miles example you are willing to spend on. Non winter/salt climate cars are much better to buy (due to underbody rust) and extensive service history will give you a good indication how it was treated since new.