You will need to explore the island anyway, so talk to the 8 candidates. If you enter the battle tower without making a choice, it will be made for you (randomly).
I think you may have missed a few things if you are only level 13, especially since the mindread in your first encounter on the island gives you enough experience to get to the next level.
Amdusias was level 15, so you should usually be about 14 or 15 by the end of Broken Valley. Exploring the start of the fjords could get you into the early twenties (I got to level 22) before going to Sentinel Island.
