I think its a wise decision,i would hand the car back for a full refund as it will spoil your enjoyment of the car as you will be obsessing about noises etc,technically he wasn't lying there is no interval for the chain to be changed it only needs changing when they become noisy and even then you could get away with just a tensioner,there are hundreds if not more on mega mileages and still on original chain there are lots of theories regarding premature chain wear some say shorter service schedules,some say turn off the stop start but it only a single chain compared to a duplex,but even the petrol engines suffer as far as I know the only one that doesn't is the v6 diesel but best of luck in what you decide to do I hope it hasn't put you off Mercedes ownership as they are really great cars.No offence taken, my budget was 7k and was really thinking to pay for the work myself but if im honest I don't trust the seller and the car 100% when I enquired about the car I asked i asked if chain had been done and he said the chain doesn't need doing on this model( which is a lie as I researched the engine on this model) I've bad luck when it comes to chains my previous car cost me 1100 to sort. I will keep looking and will take your advice on board and look to a older model. Thank you