Samsung 850 Pro 256GB for OS & programs - excellent choice!
Now for a big, fast work area... why not upgrade to the 512GB Samsung xp 941 and use that for all files: media, cache, previews, exports, projects. That should be plenty fast.
And finally, for the backups... a single 4TB should work just fine, or two 4TB HDs would be even better (1 on-site and 1 off-site). Drives do die, and with a data space of only 512GB on your main PC you may not have sufficient space to keep all your media and projects after awhile. I personally like to use "hot-swap" bays for my backup drives. If you don't have a hardware controller that supports hot-swapping, it is still not a big deal at all to turn the PC off, put in the backup drive(s), boot, and complete the backups. Then, turn the PC off, remove the drive(s) and you're back to work.
Regards,
Jim