You should use nothing but sparkplug wire for the plugs. Use only non-resistor type for a WD. I'd buy the pre-assembled type myself - it has the moisture boots on each end already. For a few dollars more, you can do it right. As for the coil: You can use 18 AWG wire if you like. Personally, I like 16 AWG better - it has more mechanical strength, if nothing else. Heavier wire will not gain anything, and is harder to work with.