Many of your purchases will likely be using credit card. But getting 100 euros at an ATM in country won't hurt anything.
A quick look around the board, or at Google Maps shows the train station at Civi to be a short walk away...6-8 blocks, not across the street. As frequently mentioned here, it is walkable, or there is a bus for 2 euros (I think), which will take you from the station to the gate of the port. You may be able to walk to your terminal, or take a free shuttle bus within the port, depending on which terminal you are at.