Christian,
Glad to hear your interested in purchasing Anker products! I think we can all agree that Anker’s product line can be quite confusing, especially for new customers.
I think the PowerCore II 20000 is a great power bank, but it will not provide fast charge to your iPhone (it won’t be slow, though).
As far as differences between IQ2 and QC3, really the only difference is that IQ2 is cheaper for Anker because it is not certified by Qualcomm. It still provides the same charging speeds as QC3, though.
The ideal pairing with PowerCore II 20000 would be an IQ2 PowerPort, but unfortunately, Anker has stated that these are not coming out until sometime in 2018. To take full advantage of recharging your PowerCore the fastest, you would need a QC3 PowerPort.
To fast charge your iPhone 8, you would need the PD spec, which is not related to QC3 or IQ2.
At the moment, Anker does not offer a charger of any kind that would be capable of outputting PD and QC3/IQ2.
As far as the cables, the included cables in all of Anker’s products will be able to take advantage of the fastest possible charge in most cases.
May I ask, do you own any quick charge enabled products, or is it just the PowerCore you’d be looking to charge with QC3?
If you do not own any other QC enabled products, I may not even recommend the PowerCore II 20000 considering your situation with your iPhone.
If you don’t mind the higher pricetag, I would honestly probably recommend the PowerCore Speed 20000 PD. A PD enabled wall charger comes included, however it is only a 1 port.
If you wish to have an additional desktop charger, there is this, which has 1 PD port (capable of fast charging your PowerCore Speed 20000 PD and iPhone 8) and 4 regular USB ports.
You will also need a USB-C to Lightning cable to take advantage of the fast charging for your iPhone.
The big downside to going this way is that these three things would cost you about $200 with tax.
A slightly cheaper solution would be to go with the PowerCore II 20000, PowerPort Speed 2, and PowerPort Speed 5 PD if you don’t care about fast charging your iPhone on the go.
Hope this helps clear up some of the confusion and please reach out if you have any more queries!