all editions from the Melodyne singletrack family (i.e. Melodyne essential, Melodyne assistant and Melodyne editor) can operate as both a plug-in (VST, AU, RTAS) and a stand-alone application (see also here
). No ReWire or MelodyneBridge required. Only Melodyne studio has the ReWire and MelodyneBridge options.
Please keep in mind that if you upgrade to Melodyne studio bundle, you will actually get two separate products: Melodyne studio and Melodyne editor (incl. DNA). So this upgrade path is the cheapest way for you to get the most advanced Melodyne technology.
However, if you are sure that you don't need neither multi-track features nor DNA, have a closer look at Melodyne essential and Melodyne assistant. See also our product comparison chart
and my reply in this thread