Which Car to Buy
Assuming that both cars have been equally well cared for, consider this. The newer car is still covered by a MB warranty. He could buy it, take it to a dealer and have them go over it with a fine-toothed comb. Most repairs would be on MB. He should be glad that it is a '99 as opposed to a '00. they made a lot of changes in '00 and I think they took some quality out of the car.
If either car is coming from a dealer, why not ask them to Starmark it first?