I faced a similar decision about a year ago and concluded the following:
Like you I really prefer the styling of the W126 cars over the W140.
The W140 series cars were the last built by Engineers rather than Accountants, but are if anything over-engineered with complex electronics and electro-mechanical components. Great if they all work, but a $$ nightmare if they start failing.
The W126 cars are not infallible, but their known 'problems' tend to be much easier and less costly to fix. Even the 560SEL with all its bells & whistles is a fairly straightforward car to repair.
Initial cost-wise you can probably buy a decent 1999 W140 for less than a good 1991 W126, as values on the latter are definitely increasing, but long term I believe the 560SEL is a better buy.
Just my $0.02

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown