Do you know whether you have the M103 or 3.2L M104-series engine? The head gasket issue is primarily an anomaly on the M104 engines.
You may have read about the wiring harness issues, but I believe that affected 92-94 models.
I am not aware of any serious electrical gremlins save for the defective OEM OVP relay, but if an unscrupulous mechanic or careless previous owner "jerry-rigged" anything in order to repair something they knew nothing about, that could cause a chain reaction of electrical problems in the future.
If the OEM radiator hasn't been replaced yet, you may have to soon. The original rads crack at the plastic hose inlet neck about 99% of the time when they do fail. The replacements have a reinforced neck with a metal insert.
Okay, you've heard enough about what could go wrong...what is right about these cars is that the bottom end is almost bulletproof, and parts are not as expensive as you may think. Plus, much of the maintenance can be performed at home with average tools. They are actually pretty reliable cars, and I probably don't service my W124 any more frequently than I do my other MBs, even though the W124 does the daily grind while the others are weekend drivers...
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