HP Zbook recommendations?

Hi,

I'm looking into buying a new HP Zbook as my Elite 8760w is getting old and quite tired... BUT I'm at lost at the many different variations available...

Besides the ram, internal drive, and minor features, I'd like to know if any of you think there are major/significant noticeable difference between their processors and graphics cards, from the cheapest to most expensive (G3 series).

Also: for some reason, HP Canada still don't carry their latest G4 models and am wondering if it's better to purchase from the US to get that "bonus" optimal machine. Yet I'm not sure if I'm going to be for their tech support. Cause as much as I had a good amount of problems with my current workstation, I can honestly say that I've been VERY satisfied by the solid post-sale customer service, and that alone makes me want to buy from them again.

In other words, which is the best unit that will save good time on the renders and live workflow productivity in PP & AE? I mean, I don't need to buy the very latest/top one as so often the extra money is simply not worth it.

Do all of them have no problem playing 4K footage in real time? Proxy making is annoying!

Check out these models and I thank you in advance for your help!

http://www8.hp.com/ca/en/products/laptops/product-detail.html?oid=8693765#!tab=models