To get more than 60p with your own camera you need to purchase the software Twixtor, which can easily double or triple the frame rate, though you must use the correct shutter speed when you shoot for Twixtor processing.
Otherwise, high quality slow-motion requires renting a specialized high-speed camera and high-speed lighting to go with the camera. These cameras are in the $20K - $60K price range, so buying one is out of the question.
There is also the Sony FS-700 camera that can shoot up to 240 fps, but it's also in the $8K+ price range to buy, so you will probably want to rent one of these.
I always try and sell local first ( I live in a big city ), and then I use eBay if I can't sell it locally.
eBay is great as long as you ONLY use PayPal for payment, so you are protected against being ripped off. ( DO NOT accept any other form of payment when selling on eBay )
For the best video image I would look at either the Panasonic GH3 or Nikon D5300 cameras.
The GH3 has the advantage of being able to shoot for 3+ hours on ONE battery, and has the 4K upgrade path with the new GH4. It can also use just about any lens ever made with the right adapter.
Cinematographers are still needed for 3D CGI work, as they often required to do realistic camera and lighting within the 3D world.
This is simply slow shutter speed. You will need a few good ND filters of different strengths depending on the time of day you want to shoot.
The new Hoya PRO ND filters are the ones I would recommend, as they use a new and better metalic coating process that blocks some of the IR spectrum so your blacks will record as black and not rusty-black. ( cheap NDs are famous for NOT blocking IR and producing sh*tty looking blacks )
A used RED ONE has lots of issues to deal with: Reliability, Weight (it's a very heavy camera), Expensive recording media, etc...
This would be my absolute last choice.
>>>Sony FS7 and a lens
This is an $8,000 camera, and adding on a couple of lenses will put you past the $10,000 point, so it's double your $5,000 budget. I would only consider this if you have the budget to cover the camera and lenses. ( i.e. $12,000+ )
>>>BlackMagic 4K Cine camera and some lenses
With new firmware upgrades the BM 4K Cine camera is turning out to be a decent camera, and this would be well within your $5K budget. It would be a great fit for digital-cine work, but not as good for corporate/commercial video work.
If you already own a Pansonic GH4, then you would be nuts to not invest more in to this system. This camera is extremely capable of producing great images, as long as low-light is not a big part of your work. There are several shooters I know of that are using the GH4 along with the Sony AS7 camera, so that they have all bases covered. ( i.e. 4K internal with the GH4, super detailed wide angle daylight shots with the GH4, low-light shallow DOF shots with the AS7, etc... )