Buying the starter set is crucial, otherwise you have no dice or maneuver templates. Plus you can't play Luke Skywalker in an X-Wing without the core set! I don't often use him now that my squadrons have grown, but I'd be remiss if I lacked my namesake in the game as a pilot.
Past that buy whatever you think looks cool. Upgrade cards you can print off if you'd like (I haven't met a person yet that has a problem with this) as you only get certain ones with certain ships and they aren't sold separately in booster packs (yet....FFG take note!). The great thing about this game is that even if you're playing 'weak' ships you still have a decent chance to win if you're better at planning and anticipating movements.
Either way, here's a handy list
of almost every expansion pack and what comes in it for easy reference.