I am not sure about item #1 but my thought is that some kits allow you to custom make your own so the lengths of wire are how you want for looks or for different applications if the car is a complete custom.
I think you should be able to assemble them yourself I have done some of my own before
You would need a crimping tool and or be able to strip the end of the wires if they are not already
8mm or 8.5mm is better if you have higher voltage ignition systems if your ignition is stock then any of those choice would be OK. If you plan on upgrading the ignition/coil in the future I would go with the 8mm if you are not concerned with originality
Your last question depends on weather or not you are concerned with originality or not? If not then they are OK.