Ask to see if the following has been changed...

1) center panel window switch?
2) power steering hose clamp (see if recall done)?
3) shocks replaced?
4) Center drive shaft bearing?
5) Aux fan harness upgrade?

These are pretty much the things that will break eventually... mostly at around 90k miles... well, at least on my 2000 ML430...
#1 ~ 75k, #2 ~ 65k, #3 ~ 100k, #4 ~ 95k, #5 ~ replaced myself ~ 60k.

I think the engine and the trans is kinda hard to test... but for the engine, maybe you can pull the dipstick and smell and look at the oil... see if they are really black, any residues, and smell burnt... if smell burnt, maybe worn piston rings; if black, may need oil change; if residue, worn engine...

Hope this helps...