@ Imtishal! Well if you are getting this City in good price (say according to the current market or even 10k more) go for it. But incase the person is asking you something out of the world kind of amout I think your should stay away.
as far as running on cng or petrol, to be honest i have been using cng cars since 2000 and i dont see much effect on engine, if the car is kept well.