There are lots of used cameras you can get in that range. With a bit of luck you may be able to find someone who's "upgraded" and is looking to part with their kit. Any of these would work great with a Jag/Letus/Brevis/Red Rock setup. Get just the camera first, do a couple shorts, get used to the workflow, then add the adapter and some lenses to give yourself a more traditional narrative film-making image and etc.
The Canon HV series might be findable in that price range used. There's always venerable cameras (but still great) like the TRV-900 or the GL-1 (Sony and Canon respectively) in that price range as well. By range I mean ~$500.
ebay is your friend. Look for stuff in your range, then google the model numbers to see if they have the features you'll want. Mostly as much manual control as you can get:
External Mic Input of some sort
Manual Focus (ring preferred)
Manual Exposure
Manual White Balance (The more programmable the better, but at least a selection of options)