I would not hesitate to pay $10k for a 100k mile 560SEL with perfect paint, upholstery and records, from a dry climate.
If you are into DIY, the key repair tasks appear on my web site and are inexpensive if you do them yourself.
Hydraulic suspension repair is easier, if it's just the accumulator spheres, than shock replacement, IMO. If the hydraulic suspension puts fear in your heart, consider the 420SEL. A little less power, generally fewer interior appointments unless they were special ordered (heated seats, reclining rear seats, etc.), a little less power, but otherwise every bit as good, plus you get 2 more mpg. Neither motor is efficient by modern standards - beating 20mpg overall would be tough.
New paint and upholstery can cost far more than an engine repair so find the best you can unless appearances do not matter to you.
Climate control problems are not too tough to fix, but it takes time - read $ - unless you DIY.
Except for the ritual 100k mile chain, tensioner and upper rail change, you are looking at a 500,000+ mile motor here.
These are generally very DIY friendly vehicles, in contrast to the ones that came after. A $10k post-1985 W126 is, IMO, a far better proposition than a $15k W140 - the reasons the latter are so cheap are extensively documented on this board.
You can decide which looks better!