Don't own a 560SEC, but I too would like one. My research has brought up the camshaft prob on higher mileage cars, and A/C evaporator or condenser(don't remember which one now). Both items are supposed to be pretty spendy on these cars.
But the w126 series are built like tanks, and last a long time if not rusted out. Might try an SD, or maybe a 300SE/SEL for something that'll do better on gas. The SDL has a spotty history with cracking cylinder heads. (Do a search over in the diesel forum.)

__________________
past MB rides:
'68 220D
'68 220D(another one)
'67 230
'84 SD
Current rides:
'06 Lexus RX330
'93 Ford F-250
'96 Corvette
'99 Polaris 700 RMK sled
2011 Polaris Assault
'86 Yamaha TT350(good 'ol thumper)