|Edited: 28/04/18 11:20
If you want to play all six X games of the old series (X: Beyond the Frontier to X3: Albion Prelude), simply buy the X-Superbox. Even if it does not contain X3: Albion Prelude on DVD, you can get it for free by activating your Superbox on Steam.
If you are only interested in the newer X3 games, you should buy the X3: Terran War Pack. This is a bundle of X3: Terran Conflict and X3: Albion Prelude.
If you want to play X Rebirth, with the X Rebirth Complete Edition (X Rebirth: Home of Light Edition Complete Edition on GOG) you get X Rebirth with its two extensions The Teladi Outpost and Home of Light.
Please note: Whatever version you buy, Updates ("patches") are always provided for free, either through our downloads section (for CD/DVD versions not bound to Steam) or directly through the download platform you bought the game from (e.g. Steam or GOG).