Funny that no one has replied to this yet. If you are looking to edit the 4k h.264 coming off the GH4 or the xavcs from the a7rII you should be fine editing off a single external spinning drive over usb 3.0. Thunderbolt is only going to be useful if you are running a raid array as a single spinning drive is not going to even come close to saturating usb 3.0 or thunderbolt. You would be spending extra money for thunderbolt for essentially nothing. If you are looking at editing 4k prores coming off the shogun that is probably going to start pushing the upper limits of what a spinning drive can handle as far as real time playback through an NLE. I know that I have 60fps prores files on a fairly fast (as far as spinning drives go) spinning external and I can't playback real time. 24fps seems to play ok with some dropped frames here and there. You can forget about the prores 4444 stuff that I have :)
What is your price range exactly?
My solution would be for the more extreme situations (shooting everything at 100fps as you had mentioned) where you are going to have a much bigger difference in shutter speed between 180degrees at 24fps and 180degrees at 100fps (1/48th vs 1/200th). The extra motion blur added in post is going to negate that strobing effect that you get shooting at higher shutter speeds/lower shutter angels. If you are just thinking of shooting everything at 48fps then you are going to run into a lot less of a headache and shooting at a 140degree shutter would probably work nicely. Obviously you wont be getting as slow of motion as your previously mentioned 100fps which could be a drawback. You can also look into frame interpolation to slow down your 48fps footage but depending on the amount you are slowing it down it can have mixed results.
An option for your shutter speed problem is to shoot everything at your desired 100fps with the correct shutter speed (1/200 would give you a 180degree shutter) and then in post add motion blur. After effects has its pixel motion blur effect which allows you to adjust shutter angle, samples, and detail and in my experience it does work quite well. Another option would be to go with the 3rd party plugin Real Smart Motion Blur which apparently is much faster with around the same results as pixel motion blur. Obviously this can be time consuming depending on how long your final project is and how much of it you need at normal speed vs slow motion. One thing to take into consideration is you can always add motion blur but you cant take it out.
See if those work. Otherwise there are a lot available online usable through your web browser.
These guys definitely gave me an itch to give this technique a try. It's no D800 but I managed to coax out some 4.5k 12bit RAW video out of a GH3 mounted on a cheap slider I have laying around. If you pay attention you can see some artifacting and stutter but in all honesty I was surprised it worked this well. It's crazy to see what the sensor on any given camera is actually capable of vs the video you get after it has been subsampled and smashed into a compressed codec. https://vimeo.com/122211023
Unless you are going to run a raid configuration or are using fast SSD's, Thunderbolt is not going to provide you any real benefit over USB 3.0. A thunderbolt drive is just going to cost you a lot more. A regular spinning drive is not going to even come close to saturating USB 3.0. Just find a decent 7200RPM external USB 3.0 drive.