Any wire you'll get from most auto parts stores today will be resistance wire. If you want metallic core wire you'll have to go to a lawn mower or motorcycle shop.
8mm wire will require trimming the outer insulation before you can screw it into the CDI, if we're talking about a Chinese 2 stoke engine. 7mm will fit better without trimming.
Many of us old timers prefer metallic core wire over the resistance wire but there'll be arguments about that so its something you need to decide for yourself.
As for the plug boot; any automotive quality boot will be better than the plastic kit supplied item and you won't have to remove the top cap of the plug to get it to fit.
If you're using the kit plug or one without the top cap you'll need to replace the plug or install a cap to make the new plug boot fit correctly.
I got a new spark plug from NAPA Auto Parts. It's part #BEL 701050. All you have to do is cut off the end that connects to another car component, and then you screw it in to your CDI. It fits snugly on to the plug and it's longer and more flexible.
It's not a solid metal core, but I honestly can't tell the difference between it and the stock wire. Seem to work the same.