I am not sure what do you drive right now, if it is a W126 diesel you already know most common issues. It also depends upon how car was kept and maintained during its life.
These are great cars but on a certain cost, if not properly maintained this could be the money pit. Most common problems are:
Sun roof machnism.
AC not working (compressor, condesor etc)
Blower motor regulator.
Timming chain and timing chain rails.
Valve stem seals.
Leather seats ripps and cracks.
Dash board cracks.
The gas comsumption is 15-19 mpg, depends upon city or highway and how do you drive. I get 15mpg on highway/city drive. These cars runs for ever, great comfort and very powerfull car. Some folks think 420sel is under power and 560sel is much better, it might be the case but I think it is very powerfull with good acceleration.
But if your friend has maintained it properly and have all the service records it would be a steal at this price. Otherwise it would be the most expensive car you ever bought. Spending aobut $100 to do the pre-inspection by a indy or dealer will give you much better idea about what are you getting into.
I hope this helps, Good Luck.
Thanks

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87 420 SEL
Looking for a nice mid 80's MB diesel.
87 BMW 528e (Sold)
99 TOYOTA 4Runner
1989 Honda Accord (Sold)
1988 Honda Accord (Total)