mercedes bad service

I have bought an mercedes a*** model and contacted mercedes with vin number to see if all contracts was active and it was a green light so i have booked the service with mercedes and all was great till day before service drive the car and there was an knocking noise in engine so i called mercedes road side assist to pick car up from my parents house engine on idle was quite and under power had a knock so did see car was prev repaired so mercedes told me they flagged car and i will have to come and pick up my car and what did i got back is shocking the piston went true the block and next day went to mercedes centurion were car was and spoke to service manager and principle of dealership and they told me its not there problem i was never in the wrong they qouted me R248k odd to replace engine and then they will lift my flag on car but my car was fine with small knocking noise now from them more damage were this could have been cheaper to repair now i have to replace so don't buy second hand mercedes even if the vin number is green like mine will take this to court now but every one needs to no what is under the covers @ mercedes benz they are bad service if i bring my car on load bed truck that means do not start vehicle and i sed knocking noise in engine so mersedes was in the wrong not me