357 is probably my favorite caliber. Sounds like you already have a really good idea of what you want. The GP100 is a great choice for a new one. A new Smith will cost more than what you are looking to spend, but there are plenty of good used ones. I have a couple Dan Wessons and they are outstanding. The older Ruger Security Six or Police Six would also be a great choice. People here keep saying you can pick those up for 3-4 hundred bucks, but I don't know where they keep seeing those. I haven't seen one at any of the gunstores I go to for many years.
Like Rob mentioned, don't discount the Single actions (unless you've already shot some and don't like them). But I'd recommend trying one out, you may fall it love with it.