You need to buy all of the ones that unlock content that you want to have. They all work on the same underlying game version, but some things work in a "default" or "vanilla" way unless you have the appropriate expansion. My advice would be to read what each expansion adds, play the vanilla game a bit to assess what would interest you, and then buy those that unlock what you're interested in. Unless, of course, you have plenty money and know you'll want it all eventually - in which case go for it!
