At 100K, if the seller is doing it just to upgrade, my take is that most of the problems have been taken care of. I also believe that if the vehicle was problematic, the owner probably wouldn't have kept it this long. I think that high mileage MLs, believe it or not, may actually be the best built ones. Do a CarFax, check the VIN via a MB dealer and if you're comfortable with the maintenance record get it. But make sure to tell the seller that final buying depends on what an independent shop says.
Definitely have it checked by an independent shop before signing on the dotted line.
Remember: at 100K, you'll need to have the spark plugs replaced.
Things to check, meanwhile:
Ball joints - they seem to go early
Steering hoses - leaky, there was a Tech Bulletin or recall on this
Coolant hoses - leaks aroung water pump or sheared hoses around radiator interface
Fuel pump - goes bad for no apparent reason
Power Sunroof dust cover - seems to come loose for unknown reasons
Catalytic Converter - there's a recall on that currently
Engine Pulley - just drew a blank here, but one of the drive belt pulleys is currently on recall
Breaks - they do go fast, so check to make sure they're OK
lights - check for condensation
There are lots more things to check and I'm sure others will chime in. But like I said earlier, at 100K, most of these have have probably been taken care of. If you want more info on what to check go to this somewhat over-exaggerated post:
http://forums.mbworld.org/forums/showthread.php?t=88443
Take it all with a grain of salt, lots of it!!!