Just buy Rebellion...
SoaSE (the original or vanilla) was released back in 2008 (I assume that's what you mean by "first")...a year later, an expansion called Entrenchment came out...a year after that, another expansion called Diplomacy came out...up to that point, Sins was installed via Impulse, a download client which at the time was owned by Stardock itself (one of the developers of the game)...some time after Diplomacy came out, SoaSE Trinity was released....Trinity simply combines all the features of original, Entrenchment, and Diplomacy into one package....if you do wish to buy something from before Rebellions, buy Trinity...
Trinity can be purchased via Impulse, but Impluse was sold to Gamestop...Trinity can also be purchased via Steam...
Rebellion is stand-alone...it includes all the features in the original game as well as those added by Entrenchment and Diplomacy...if you only own rebellion, you will not be missing out on anything...Rebellion requires Steam in order to play, so while you can buy Rebellion from other places, ultimately you will have to have Steam regardless...so, you might as well buy Rebellion on steam unless some other place happens to have a lower price...
I personally would suggest buying only Rebellion, and purchasing it via steam...as for Italian, I don't know what to tell you...at some point in the future other languages may be added but that is unlikely, and Italian may not end up being one of the languages added...you may want to see if you can simply convert some of the in-game text to italian...it is a pain but it is possible...
