I think the easiest one to use is a drill with a cutting bit -- not sure if they come in a 120mm size, but there's not a whole lot else that's as easy to use. Dremels are nice -- and after using both, i still prefer the drill for circular holes in metal sheets. Definitle might want to get some cutting fluid -- you don't need a whole lot -- but the difference it maes on the life of your cutting bit, your drill and the cleanliness of the hole is well worth it.
Just my 2 cents.