upgrading to the xeon's has the benefit of keeping your existing ddr3 ram, OS, and easier hardware installation. however the newer hardware with ddr4 will be slightly better for 4k+ video editing and a single cpu system won't have to deal with a slower dual cpu interconnect. microsoft has decided to play dirty in order to force people into buying win10 and won't offer updates to win7 on newer cpu's. if you don't care about having the latest win7 updates, you might be ok with running win7 on new hardware.

if you can wait a couple of months, there are rumors intel is releasing x299 soon. rumors of a 12 core i9 (i9 replacing i7 for the very top end cpu's), and possibly a price drop for the rest of the lineup. so the new 8 core might be slightly cheaper. the i7-6900k is broadwell-e and doesn't overclock past 4ghz very well, but the new cpu's coming will be skylake-x and should overclock a bit better. x299 will have higher base memory speed support, as well as higher pcie lane count and pcie 3.0 support on the chipset, so a nice bonus all around.

you are right about pcie/nvme m.2 drives not being very beneficial for the OS drive. sometimes they can help with very high bitrate media and sometimes with cache situations. there are pcie x4 m.2 card adapters that allow an m.2 ssd to plug into a pcie slot on the motherboard, so you won't need a motherboard with multiple m.2 slots. its possible that you could run an m.2 on your existing motherboard with a pcie adapter card too. the rumored x299 motherboards are likely to have 2+ m.2 slots on them, and there was even one with 3 slots.

also just a heads up, amd is also rumored to release a platform to compete against intel's x99/x299. those cpu's are rumored to have 10-16 cores, and will likely be more affordable. the downside is the current amd cpu's don't overclock well past 3.9ghz, and may not be as responsive for ultra low latency audio daw work. if you are curious about the amd cpu's you could research for DAW reviews/feedback with the current 8 core ryzen cpu's, like the ryzen 1800x.

Of course one core at 4.3 GHz would beat one core at 3.8 GHz.  And with the newer motherboard you get one super speed M.2 SSD slot