Harness was one of my big ($$$) problems, timing chain the other.
Get an inspection done by someone who knows these cars, they can get expensive quickly.
I bought one with 150K, wife loves it (except the mpg!), did the chain ($$$), the harness ($$$), and replaced the ignition switch (124's seem to have a quirky switch, which if it gets stuck anywhere but off position has to be drilled out $$$) .
You can tell a lot by the condition of the interior and the engine bay. The closer to showroom, the more likely the car has been taken care of. Try the fan on all settings, listen for squealing. Try all the windows, check the lights on all the guages and vent knobs (not all expensive, but a pain!). Try the sunroof, see how smooth and how it sounds. Have someone turn on the lights and use the window washer and watch the headlight wipers for functioning.
Pull up the carpeting and check for moisture in the padding (we haven't had any problem with our 124, but my wagon... grrrr!).
Crank up the stereo and check all the speakers. They are pricey if you want original.
Check prices here, and don't pay too much! You'll need the $$$ for premium fuel!
