With video editing and rendering, more cores == faster. Also you'll probably find that the professional software programs require professional cards for rendering/GPGPU offloading. That means Quadro/Firepro cards.
Since this is your job, you don't necessarily want to skimp, time is money, but you can buy carefully.
To be honest, you could save a lot of cash going slightly used on the cpu and mobo. HP Z800 dual socket mobo, new, $80 on ebay, dual hex core lga 1366 Xeons, $70 each. Thats 24 threads of processing for less than $230.00 and from a benchmark pov, a single Xeon x5650 is about 10% slower than a single i7-5930. But you'll have two of them, and a mobo for half the price of a single 5930k cpu alone. And that will blow the single 5930k away.
The Z800 mobo supports a bunch of RAM - 12 slots, and you can get 12- 8GB DDR3 DIMMS for $250.00 (96GB total).
Then a case, PSU, and drives.
As for the graphics, the money you've just saved on the main system you can put towards the GPU. The M4000 is a good card, based on Maxwell, 8GB, and brand new is $800.00 Two of them would run $1600.00 . A single M5000 is about double the price, the only difference is more cuda cores, and (which may be important) has ECC. Since you're not doing scientific calculations, the ECC may not be critical, as video rendering is a little more fault tolerant. So - I'd say a pair of M4000's would be more cost effective, and again (like the twin X5650 Xeons vs single i7-5930k) you have twice as many slightly slower GPU's at a cheaper price.
So - that's what I'd suggest.
HP Z800 Motherboard (new, ebay) -
http://www.ebay.com/itm/NEW-HP-Z800-Workstation-Motherboard-Dual-LGA-1366-Sockets-460838-002-576202-001-/161815724766?hash=item25acf802de
two Xeon LGA 1366 X5650 Hex Core (used, ebay) -
http://www.ebay.com/itm/Intel-Xeon-X5650-SLBV3-2-66-GHz-Six-Core-12Mb-Cache-LGA-1366-without-fan-/252029437274?hash=item3aae20055a
96GB DDR3 ECC Registered (used, ebay) -
http://www.ebay.com/itm/Lot-of-12-8GB-DDR3-8500-10600-12800-ECC-Registered-Server-Memory-DIMM-RAM-/252077289668?hash=item3ab0fa30c4
Two PNY Quadro M4000 (new, Newegg) -
http://www.newegg.com/Product/Product.aspx?Item=N82E16814132051&cm_re=quadro-_-14-132-051-_-Product
That would give you a pretty killer system for around $2100 for the basic core system. You'd still need some miscellaneous things like case, CPU heatsink/fan, drives, fans, and PSU. But it would be a little beast of a video/rendering machine. ECC ram, 24 threads, 40,000 rough geekbench score.