I will ASSURE you that you will be much happier with the 560SEL than the C-Class. There is simply no comparison between the two when it comes to road abilities. Sure, the C is more agile but NOTHING will top the long wheelbase 560 in highway cruising.
I've worked on many many 560SELs and other various W126 cars. The thinking that trannies go out at 125K is a MYTH!! I know plenty of cars running around out there with well over 200K behind their trannies and still going strong. The last car to have transmission problems because of the high torque of the engine was the 450SEL 6.9 (I know firsthand. I have one).
The engines are near bulletproof. We recommend replacing the timing chain, tensioner and upper guide rails at 100K intervals. Oil leaks aren't uncommon from the rear of the right hand cylinder head, sometimes necessitating a head gasket replacement.
The rear self-leveling suspension is pretty trouble-free but the accumulators are finite life components and generally need replacing around 100-150K miles. They act as a springing medium at the rear end. The struts are the actual leveling devices. Sometimes the leveling valves can leak, but not too often.
Caster joints and ball joints usually begin to go around 130K miles so listen for any odd clunking or grinding noises from the front end when driving over bumps.
The automatic climate control systems can be troublesome, and the A/C was always a Mercedes weak point. If there is no heat, the monovalve insert is most likely at fault. A $30 part and 10 minute job to replace.
Rear window sliding jaws break often, they're of nylon construstion and just wear down with age and use. Electrics throughout the car are 100% reliable.
W126s don't rust badly unless they've been neglected and have been wrecked and badly repaired.
Also, one last thing to consider is the un-economical operation of such a big car. But it'll be worth it every time you're behind the wheel and cruising at 120mph with the A/C on and some Mozart on the hi-fi.