560 SEL's are great cars. 124,000 miles is not a lot of mileage for this car.
What's most important is maintenance. You need to take this car to a Mercedes mechanic and pay for a PPI (pre-purchase inspection).
The most important thing about this car is the timing chain issue that Snibble mentioned. The interval to change the timing chain, tensioner, and chain rails depends on how often the oil was changed, but generally it should be replaced at about 120,000 miles.
The price also seems to be below market. If the condition is right, there are people that would pay $8k-$10k for this car. KBB usually underestimates the value of these cars.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".