I would tentatively venture a cost of 4, or making it "2, Tap" and nixing the stealing (only attaching to creatures with the same controller, much like the Simic Guildmage [link]). The "best" costs will probably have to be determined via testing.
That's a very good ability, which can perform two functions. It can move around heavy equipment like Argentum Armor, or better, it can instantly move things like Lightning Greaves to protect creatures. I think it's costed ok.