For $3000, you are not going to find a cream puff, so expect a bit of repair and maintenance to run in the thousands, until you get the aged mechanical components replaced.
As earlier stated, climate control is a royal pain in the $$$ if servicing is needed there.
I would bank on the diesel models for reliability and longevity. The S class models are superbly built, but they will drain your pockets on repairs.
You could find a later model MB for $3K with body damage, but because of the way MB designs crumple zones, I would stay away from "severe-impact" vehicles.
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