Hi DT,
If it were me (and it was), I would go for a "proper" high backed version as some of the ones in that link aren't high backed to me. This is a picture of my 8000 (with head rest obviously) and the back of the chair should come level to just about the base of your neck (I'm about 6ft btw).
I have bought a few of these chairs (for myself and family members) and to be quite honest there's not a massive amount of difference between the 6000 and 8000 series. Just don't be swayed by model numbers, go for the the one that is most comfortable to you.
The 8000 has a slightly better forward tilt mechanism, but I prefer the head rest on my 6000 series as you can get it out the way if you want. In terms of things to look out for, just check all the adjustments work and spend a good 5-10 minutes playing with it to check you're happy with it.
Any more questions, ask away