You can de-build the wheel but it will be annoying,but will make drilling alot easier.
I would use a 6mm or a 7mm then go over with a bigger one to countersunk it that looks good.
normal drill bits will be ok aswell.
You can drill every hole with like a 4mm first so you get it correct then go over it.
You could measure or just draw on the back of the rim where to drill.
Is the wheel the £45 onesfrom tartybikes the basic ones ?