the W126 chassis is still considered to be one of the best that mercedes ever made. With the coupe you get some small issues such as rear window regulators going out, and the seatbelt extensions going out, but thats pretty much it. Oh yeah, you will want to inspect the timing chain and timing chain guides too. Other than that those cars are pretty damn close to bulletproof. You are correct to be scared of the maintenance on the 750il's. I love BMW's, but their 12-cylinders are just not a motor to own out of warranty. If you want a bimmer, look at the 740il's, they still have plenty of power but you won't have a potential 6-12k repair bill looming evertime you start the darn thing.
Just my .02 cents
Ryan

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83 300SD.......sold
96 integra SE....sold
99 a4 quattro....sold
2001 IS300.......sold
2002 330i.........current.
2004 highlander limited....current.