Anyways I was thinking about just getting a really thin, packable jacket, but now I'm wondering if I should look for a more insulated one that would also feature a more breathable material. I hate being cold, by the way.
Don't get an insulated one, you'll be too hot in that 50 - 55 deg rain. Get a light breathable one. Most of the high end gear outfitters, like north face & marmot, make one that weighs about 8 oz.