Fast + Economic HDD Set-Up for 4K Editing (~2TB material)

Hello,

I have a iMac 5k, 27in, Late 2015, 4 Ghz i7 with 24gb of memory, 512 SSD, and AMD Radeon R9 M395X.

I am beginning post on a 4K doc and am trying to figure out the most economical HDD set-up that will allow me to playback and edit in 4K if possible. I have around 2TB of material, so editing from the internal SSD is not an option, unless I use proxies (haven't tested how much space they'll require).

Recommendations for an optimal HDD set-up on a budget? 2TB SSD in a Thunderbolt or USB enclosure (B&H has 2TB Samsung 860 on sale right now ... would that do the trick?)? Building an array (RAID 5?). If the array is the recommended route, I could really use recs on how to build an affordable set-up (Do I need to go with OWC ($$$)? Is there a way to do this that doesn't set me back for *well* over $1000 with the enclosure + drives?)

Or... do I generate proxies and throw those on my internal SSD if I have room (again, haven't tested how much space proxies will take) or one of my 4TB OWC USB drives if not? Once I'm picture locked, if there any easy way before/during online conform to move only 4K footage used in film to internal SSD for color correction?

Thanks for your help! There is a lot of info on fastest/economical HDD set-ups, but most seem to only be relevant if you're working with less than 500GB of material.

M