Fair enough! However, no one buys an S4 for economy people buy it to have 'fun'. ML350 is a nice SUV but it won't be cheap to run either. S4 will depreciate as much as any other second hand car and ML won't be any different.
Most of the 'main' issues with your S4 have been fixed now. Going forwards the maintenance on your S4 including the potential of other things going wrong, the probability of it happening won't be any less than that of any other car. If you read forums (not just ASN but BMW and Mecs etc) people are having issues with brand new factory cars as well.
All the best with your sale buddy.
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