With temps in the single digits I'd say NOW is great time to move down there. However, I also agree with all of the others that say to do your research first. I Live in Minnesota and would love to be in SP instead, but can't feasibly work it out either monetarily or due to personal responsibilities that hold me back.
As far as cars, yes there are a few cars on the island. Most travel using bike, golf cart or by walking. There are also taxi's for the occasional "need" to make a longer trip. Many people consider cars to be unnecessary.
With regard to other things to do: There are so many ways to get involved in the community that you could keep yourself in perpetual motion and totally occupied. With that, the social life, the diving, fishing and boating, dining, mainland trips...and the list continues, you should not get bored easily.
btw, she would be someone to get to know - in my opinion. She would give you straight up answers.
Good luck!
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