I need help/advice
From reading the forum posts, V6s seem generally more reliable than V8s. Avoid the diesel Bluetec due to exhaust aftertreatment-related problems.
If you are targeting a GL450, a V6 was available in 2015. The W166/X166 platform seems to have about average Mercedes reliability.
What are your vehicle requirements? Do you need a third row seating feature? If you do not need third row seating, have a look at the W166 GLE350 2016 and newer. It offers a reliable, non-turbo gasoline engine and proven 7G-Tronic transmission. It's about as reliable as a Mercedes can be in the current era.
I hope this helps you!








