May I suggest you to take a look at GoG too? Currently there X3 Reunion costs less than on Steam, while the Terran War Pack costs around 18 € but includes both Terran Conflict and Albion Prelude (mind you these are not dlc but stand alone games). With the added plus of no DRM.
If you're looking for the definitive sandbox experience, go without a doubt for TC/AP. Reunion may have a somewhat "more compelling" main story (big words to use, story mode has never been Egosoft's strong point, to be gentle), but the two semi-sequels added literally a metric ton of new stuff, ships, sectors, gameplay mechanics...with no official word yet on what X4 may offer, TC/AP are still the sandbox space games to beat for me.
(even though Rebirth as of 4.0, if you're able to get past the single ship thing and the embarassing npc/dialogues, can actually be a pretty neat title)