You don't have to buy anything besides the original game. The first expansion, (Desert of Flames), is for characters lvl 50 - 60 and you won't get there for a while. The second expansion, (Kingdom of Sky), is for characters lvl 60 - 70 and you won't be there for even longer!
Besides full expansions, there are also two adventure packs available which are like small expansion modules. These are also optional for you right away since they are for characters lvl 20 and above.
This is not to say that it will take you long to get to lvl 20 but only that even then you will have plenty to do without adding an adventure pack!
All that being said, there is a new package coming out in February that will include the original game and the 2nd expansion. That is probably your best deal right now.
To purchase everything you would probably looking at $65 - $90 all together. The exact amount would depend on where you bought the game and 1st expansion. The 2 adventure packs are $5 & $8 and are only available as digital downloads from SOE.