Opinions on this e420

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Hey guys I tested this e420 today it is a 1997 with 83000 miles on it
the interior and exterior are in almost perfect condition ,it has all the options except it is not a sport.
When i check the engine I could smell engine degreaser like they just wash the engine but there was still grease around the valve cover gasket dirty!
As i was test driving the car when i was taking off the car hesitated a little bit the guy told me that it was because they just wash down the engine My question is should the engine hesitate even if it was just washed.THe asking price was $15000 when i told him 13,500 he did not want to go for it but when i told him I was paying cash he said he was going to talk to his boss and that I should call him tomorrow so what do you guys think.
thanks