To unlock the Historic Rally events, you will first need to complete 15 different events in the other categories. Once it's done you will earn the license that will allow you to play in Historic Rally. Complete the available events to unlock other events, until you're able to play the last event which will be the Historic Legends Series. Once a new set of events is unlocked you don't have to beat the remaining events in the previous one if you don't want to. While progressing through your career to unlock the last event, you will also earn:
The Historic Legends Series has 5 events and every event consists of 6 stages. You don't have to win every stage/race but you'll need to have the best overall time, win the final or the A-final in every event to get the trophy.