We are thinking of putting together a new machine to do editing using Media Composer. The machine would be used to edit UHD and 4K material...

Would this be okay (All links are here to give you access to more information):

Motherboard : HP Z800 Workstation Motherboard (http://www.compeve.com/index.php?main_page=product_info&products_id=981)

Video card : NVidia K5200 (https://www.ebay.com/itm/Dell-Nvidia-Quadro-K5200-8GB-256-bit-PCIe-x16-Computer-Video-Graphics-Card-R93G/292449799000?epid=1737895855&hash=item44175dcb58:g:aJEAAOSw9mlahIzv)

Processors (2 of them) : Intel Xeon X5570 Nehalem 2.93 Ghz (https://www.newegg.ca/Product/Product.aspx?Item=9SIAA404M68104&cm_re=xeon_processor-_-9SIAA404M68104-_-Product)

Memory : Kingston Fury 16GB DDR 4 DIMM (4 for 64 GB Minimum) (https://www.bestbuy.ca/en-CA/Search/SearchResults.aspx?type=product&page=1&sortBy=relevance&sortDir=desc&query=dimm+memory+16gb)

Internal Hard Drive (For Sotware) : Seagate FireCude 2TB 3.5" SATA III (https://www.bestbuy.ca/en-ca/product/seagate-firecuda-2tb-3-5-sata-iii-desktop-internal-hybrid-drive-st2000dx002/10604153.aspx)

Further to suggestion, OS Drive would be a SSD: Silicon Power S55 960GB SATA III SSD (https://www.bestbuy.ca/en-ca/product/silicon-power-s55-960gb-sata-iii-internal-solid-state-drive-sp960gbss3s55s25ca/10671228.aspx)

We will then be adding external RAID drives for all video storage (If you have any suggestions for this, I would appreciate it)

*** Edited to correct link to Hard Drives

You posted the memory twice.

However don't let the whole speed drop down from choosing anything else than an SSD for OS.

If you want a quick machine I'f go at least HP Z820 or 840 the 800 is getting on now.

Re the RAID don't be tempted with RAID 5 go RAID 10 if you need protection or RAID 0 if you just want speed.

Wouldn't bother with the Z800.  That is a seriously old piece of kit now.  Also limited to PCI-e 2.0 so any new graphics card/interface card you put in there that uses PCI-e 3.0 will not benefit from extra bandwidth.  Z820 is the first of the Z line to support PCI-e 3.0 as far as I'm aware.

Also, the memory you have chosen is completely incompatible with that Motherboard.  Honestly I wouldn't be trying to build some sort of home-brew workstation with a Z800 as the base.  You need to do much more research in to what components work together.

Seagate drives are also known for high rates of failure:
https://www.backblaze.com/blog/hard-drive-stats-for-2017/

As others have mentioned, it's SSD or bust for the boot drive.  Time for a re-think methinks!

Look at Supermicro workstation boards as a starting point, either dual or single processor.

I can see you've added an SSD in there. 

1) It's a SATA based SSD - you will get much better speed with a PCI based SSD. 
2) The Z800 only supports SATA II - up to 3 Gbps, so you won't even be getting the full speed of the SATA III drive you have linked to.

Honestly, the list you've put together is both underpowered and more importantly, the components aren't compatible with each other and you won't get the thing to work.  You'd be much better off just buying a used Z820/Z840 from ebay that already has the memory, HDD, OS, etc installed. Then if you really want you can pimp it out a bit higher powered components, but at least you're starting from a solid base.

If you haven't built a PC from scratch before - which it seems may be the case, then doing so for a workstation that you want to run your business from is not a good idea, not to mention the fact that if it won't be a supported spec as recommended by Avid.

Take a look at the Avid supported configurations list and see what hardware you need in order to be safe.

The total price of all the components you have listed will easily buy you a solid HP Z workstation that is Avid certified and will just work straight out of the box.  Please don't go ahead with above build, it will definitely cause you problems.

Hope I'm not coming across as pushy or anything but I'd hate to see you waste your money on an old machine with outdated hardware, especially as you say you want to cut UHD/4K material on it which will require a good whack of power.  You can pick up fully built Z800's on ebay for next to nothing nowadays so if you really want to go that route then just buy one fully built.

My 2 cents

Andi