The transistion from the Roman Republic to the Roman Empire is called the Imperatorial Period. The storng leaders, or Imperators, took conrol of the government. Many of the most famous names in the ancient world come fromt this time.  Some of them include, Julius Caesar, Brutus, Mark Antony and Octavian. Coins from this era can become expensive with the notariety of some of the depicted persons, however the non-portrait types can still be affordable.