Tools -> Options -> Transfer -> tick "download pieces in order". Then you can just open the file(s) in your media player of choice as soon as the first piece is downloaded. This functionality comes from an unofficial patch for Vuze that was integrated to the mod by ghostfucker on 2014.
Note that the BitTorrent protocol's design disfavors sequential downloads, and you should avoid them on small swarms (see
Sequential downloading is bad - VuzeWiki).
Not sure what this is supposed to mean. The attachment can be found in the first post. You must install BiglyBT via the official installer and then apply the mod as usual.