hmmmm.... half mile swim? I'd probably go with the wetsuit. Anything to help flotation is something I'm on board for. It'll take a wee bit longer to pull it off in transition, but if you think it's worth it for the security/flotation then wear that puppy. If you don't feel like you need it, the water should be warm enough that you'll be fine. That wasn't very helpful, was it?

And yeah, arm warmers for sure. Sounds like it'll be cold on the bike if the air temp is only going to be 50. Yikes! I've done that before- it only looks a little weird, but who cares? Comfort comes before fashion in my book!