I'd say definitely get the heater. Here in SoCal I think our winters are a tiny bit warmer than yours, and I had trouble when they were unheated. Just set the heater to something low, like 74, so that that's the lowest it goes in the winter, but it'll rise in the summer and give them that nice seasonal change.
I can't remember, but I believe turtles are able to process dietary D3, meaning if you supply it, you don't have to get a lamp that produces UV. Don't take my word for it though...