We strongly advise that the bride and groom purchase their wedding attire in their home country. Generally, Italy does not offer the opportunity of purchasing a wedding dress on the spot; most gowns are ordered even up to a year in advance and have to be fitted, which requires several months advanced planning.
Before purchasing your wedding dress, you should first determine what style wedding, the type of location, and ambiance you are wanting to create for your Italian wedding event. Is your wedding going to be outside overlooking the ocean or will it be held in a medieval castle? Certain styles of dress are more suited for a church ceremony (most Catholic churches forbid strapless styles for example) or a very formal wedding reception while others are ideal for a less formal event.
Renting a tuxedo is not a common custom in Italy where most grooms and other members of the wedding party purchase their wedding suits outright. You should make arrangements for a tuxedo and any accessories prior to your trip in order to avoid any disappointment.
We also strongly advise that you do not ship your wedding dress in advance to avoid any mishaps and customs issues - take it with you on your flight.