Whoa, whoa there!
I have a 99, and although I don't put many miles on the non-daily drivers (the ML is the weekend hauler), I haven't had problems that would be considered a candidate for the lemon law. I will concede that the fit and finish isn't up to par when you compare to other MB series, but it does serve its purpose well. Owners of these and earlier series complain about the cheap plastic components (which MB responded in later years by upgrading said parts), but I traded up from the first model year Honda Passport (which also served me well) so the MB was a big jump, no matter the quality.
The only real concern I could point out to you is IGNORE THE FSS INDICATOR!!! Change your oil as you would normally by the seat-of-the-pants method...3000 miles, 5000 miles, whatever. If you go by the little telltale spanner wrench that appears on the odometer, you will go about 15,000 miles between oil changes...a bit lax for a vehicle you may care about keeping.
I guess you take your chances as you would with any used car...the quality of the relationship will be partly based on the care and maintenance of the previous owner.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car