On A/C's specifically, it's the evaporator that needs replacing...expensive by itself, but the labor for removing the dashboard can eclipse the cost of the replacement. Sometimes the control unit can go bad, but this is not a modal failure. The blower motor and some vacuum actuators can go bad, causing vents not to open correctly, etc. Again, this is on an idividual basis and not a W124-wide problem.
Oil leaking at the rear of the head indicates impending head gasket failure...this is for the M104-series engines. For the same engine, oil leaking at the front is due to the timing cover seal leak...much less bothersome than a head gasket. Note that the head can leak for many miles before ultimate failure...I have watched my leaky head closely for the last three years, but no breach just yet...The M103-series motors have valve stem seals that need replacing on an aging motor, and excessive oil consumption is symptomatic of this problem.
Wires can (and do) get brittle over time, especially when subjected to extreme temps, like the underhood. Most W124s had the bad main harness problem replaced, as the problem would creep up somewhere between 70-100K miles. Still other ancillary wires suffer the same fate...I just fabricated a new harness for my oil sender, oil level, and starter solenoid ground, because the OEM harness had deteriorated to where 60% of the harness was devoid of insulation!
Overall, these are pretty solid cars. Lack of proper maintenance is usually the demise of most of these cars. Needless to say, nobody sells a car because it "ran too well", so assume that a prospective W124 up for sale is probably suffering from some sort of neglect.
The good thing is that most of the replacement parts are relatively inexpensive, that is, you could pay as much for comparable Camry or Accord parts.
My final tip would be to make sure the expensive stuff was dealt with. Wiring harness ($600+), A/C evaporator ($1800 incl. labor), and head gasket ($1200-1600). All other issues if you find a reasonable mechanic, or like to DIY, should keep you in the $500 and below range...
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