Well if its a completely working cluster just used it should work no problems.. and as long as its from the same year c-class as yours even better, basically its plug and play.
as for showing your mileage on the new (used) cluster.. it will show whatever mileage was on his car.. but there is a way around this.. it means opening up both clusters and swapping just the gauges over to your cluster.
why you need the cluster? does it have white face gauges in it????