My dad has a 2002 ML320
You are right when it comes to gas guzzling. It's insane. But he loves the prestige mark so he doesn't care......yet.
We got it used with 52k km, around a year ago. After few months of driving, we noticed the oil leak. Dad took it to indy shop. They told him it was oil sensor leaking, so my dad took it to the dealer (town 100 km away)
After inspecting it, they agreed on the problem. They said they would need to drop the oil pan to access the sensor. So they did that and my dad paid nothing (still under warranty)
A month or so, leak appeared again. Dad took it in, they said one of the bolts on the oil pan was a bit loose. They tighten it.
A week after, leak (this time alot smaller) appeared again. Dad took it in and said to fix it. They agreed to pull the oil pan off and reseal again. It's been few weeks now with no oil leaks. I think they got it right this time.
While they worked on the oil leak, they also discovered that on of the ball joints, both rear springs rubber mounts and left tie rod end were shot They replaced all of it under powertrain warranty but the tie rod.
We bought the part for $28 US and my dad will take it to indy shop to have it installed, since dad and I coudln't take off the old tie rod end this past weekend.
So far, few problems but it didn't cost my dad anything really. I read that 2002+ are really reliable when compared to older models.