Athony,
All of these systems will need some hardware upgrades- RAM, drives, possibly the GPU. What is your budget for the finished system?
If you amenable to a bit more of a project, for example a CPU change, it may be desirable to purchase a low specification system at a lower price.
My first, generic choice would be to buy the z620 as it has the most potential for expansion. The E5-2660 could be tried first and a CPU change could be done if the performance was lacking. Video editing is very demanding. In that case. I would suggest changing the CPU to a Xeon E5-2680 - which is 8-core @ 2.7 /3.5GHz (about $100), add RAM to have 32GB (about $120), and an SSD 250GB of more- (about $100), and the minimum GPU I'd use in that kind of system would be a Quadro K620 (about $110) to have a least a moderate 3D capability. That would be a versatile system with some 3D capability and of course a 2nd E5-2680 could be added (requires a CPU/RAM, fan riser board- about $150 + 2nd CPU- +$100). The wild card is that Premiere may run about as well or better on a single 8-core t as it appears to peak at about 5-6 cores abd actually slows down with dual CPU's. In that case a z420 might be the better choice.
Here's an auction in which the buyer can choose components:
> and a system with an E5-2680, 32GB of RAM, 1TB drive, and a Quadro K600 costs $545.
Cheers,
BambiBoomZ