Help - which motherboard do I need / is best for my setup

No you can go with that, others enjoy it well. Looking it over I WOULD suggest you get a SSD as the OS Drive, and ask why your getting 3 - 1TB drives? Are you planning to put them in RAID 3 hot swapable?? As your doing video editing, I would suggest just getting a 4TB drive instead as storage, then a ext 4TB as backup. Also for video editing more RAM is always good for encoding/decoding in realtime, so if you could spring for 32GB instead (I think the max for the z87) and this is your JOB, then it would only benefit you more to do so (as this is your work related tools, hold all receipts and speak to a Business Tax Specialist, as the cost of this PC can be refunded in part on your taxes).

SSD: No Hype, actual performance assistance does incur especially for applications and OS layers. I was suggesting a simple OS booting one, say 250Gb usually around $100US. Load the OS and your applications (not games) onto this as your C, and you will have almost instant 'click load' applications, with OS load times around 7-15seconds LITERALLY (I have had mine for couple years now). As time does cost your money, I would HIGHLY suggest this.

3HDDs: mmm I think this is overkill, because honestly you wouldn't use a whole 1TB as a cache drive, since all three drives are still connected to the single I/O chipset, so there is no performance benefit splitting among the larger drives (your 500GBs are doing fine your saying) and would only SLOW / Impact your system more having to scan across a large HDD for small data over multiple drives. Hence why I asked about RAID (minimum drive configuration is 3 drives, 2 used for the RAID one used for data integrity). As your worried about 'disaster scenarios' (as you SHOULD BE!) then RAID would be the better answer, as you want to ensure 100% data integrity AND availability 24x7, if you go single drives and ONE decides to start failing, all sorts of 'issues' may occur which do NOT say 'drive is failing' but give false 'problems' all because where the data resides is corrupt/failing. With RAID and being 'hot swap-able' your ensuring data integrity with the 3rd drive, as well as being able to 'immediately' swap out a bad drive while working (no downtime) and it will auto restore the data across the drives. I would recommend this HIGHLY, even if you did go 3 1TBs and didn't need that much space, the benefits still outweigh the 'excess'.

Honestly every Video Editor whose posted to the forums says the same thing over and over, More RAM GOOD SYSTEM. So if you can splurge for it I would personally, as this is your business, the more money you invest the more capability to make more money (think of it this way, who makes more, a Taxi company with 16 Cabs or a Taxi company with 32 cabs? Still same amount of people out there, but more cabs, means more 'opportunities' to do more work for more people out there).

As for the 'Refresh', I wouldn't invest in it, I would stick with the 4770 unless your planning NOT to use this as a work system but as a geek enthusiast system to test and BREAK with with OCing. Note http://wccftech.com/motherboard-manufacturers-may-allow-intels-devil-s-canyon-cpu-support-on-z87-motherboards/ says the suggestion on the Z87 being usable is ONLY A RUMOR of only a couple days ago. Personally I go with tried and true then higher risk if I am dependent on it (like work or my 'main system'). I would just pair the Z87 with a 4770 and call it even.