How is it customary to pay a video editor - global or per-hour?
My experience is that it really depends on the situation and the editor.  I personally have done both hourly and project rate editing.  Typically I'll only take project rates if it is clear how much time it will take (ie, a simpler project), but it will really depend on your area and what particular editor you are working with.

I'd suggest having estimates of what you are willing to pay for either option and then simply ask them which method they prefer and then supply them with that number.  If you have a preference one way or the other, you might also ask them if they would consider the method you prefer.