How slow are you trying to go? 60fps? 96fps? 120fps? Will you be using this feature often? I personally do not utilize slow motion very often. I have a GH4, but when utilizing 96fps I take a quality hit. Albeit, an acceptable one.
If you're looking for something faster than that or cleaner, I'd highly recommend renting a camera specifically for this purpose.
There's also the very bad rolling shutter to consider with the Sony A6300. If it won't affect their shooting style, then not a big deal, but if they do a lot of movement, they'll need to stabilize it somehow.
GH4 is tried and true for me. I highly recommend it, Eric.
I may have something for you. Send me some contact info and I'll see what I can do. Thanks!
I'm curious to hear about your results. Any particular older mac pro model that would be best to use?
What are you shooting with? The specs seem to indicate that the PIX-E5H is limited to 60FPS in HD. The Ninja Assassin can do up to 120FPS in HD. If you plan on shooting any slow motion, that may be a bottleneck for you. Not choosing one over the other. Just giving you a heads up on something I noticed.
Can you set the camera to shoot at 24 frames per second? I see that it is capable of 60fps, but not sure if you can modify that. That'd be your first step.
Second, the sensor is very small, so in order to have control over your Depth of Field you may have to shoot long. In this case, I'd only use the optical zoom (not digital) and move farther away from the subject and zoom in on them. Then when you focus you should have more control over the depth of field. Otherwise, you're gonna be looking at a fairly flat image. That's fine in the cinema world, but can take away from important scenes where you want focus on one particular subject or object.
Aside from all that, the biggest thing that will help you with your film look is how you light your scenes. Definitely make that your strong point if the camera is a weak point. I'm curious to see what you come up with. Good luck with everything!