I would not waste money on powder coat, just paint it. Also if you are thinking about PC the inside, DON'T!!!!! What they don't tell you about powder coating is that it is actually plastic, and if you use a normal home oven to powder coat, you can no longer use that oven for food. As for holes, and cut edges, do those first. They will leave a rough edge that will need to be sanded down.
I would really reconsider the powder coating thing and just paint it. Paint cost alot less, and if you scratch it is easily fixed, if you scratch powder coat, you have to re-coat the whole damned thing. I have powder-coated my computer builds for years, and it is very impractical, and very expensive for a UDS. You may want to research it a bit more.