marble mantel polishing and mirror

Hi We are buying a used italianate marble mantel to install in a house we are renovating. The mantel is in the process of being put together. We now have to decide whether to have it polished or install it as is. We don’t know anything about polishing marble mantels and what the benefits are. What does polishing the marble do? Just make it shinier? Any other advantages? Any ideas what the cost is to polish a mantel like the one in the picture attached? Any places you recommend we take it for polishing? Separately, we are also looking to buy a mirror to go above the mantel (something like the one in the attached picture). Do you have any suggestions for us re where to get such a mirror? Thanks Sam