I've been into X-wing for a year or so, and understand the need to buy multiple ships as they are released. Saying that, with IA what is needed to get into the game in a reasonable way. One starter set and a couple of each expansion. Or is it only needed to buy one of each expansion kit ?
Newbe Question, how many upgrades needed?
Depends on what you want to play.
If you want to play campaign -> just get the core set and play 1-2 campaigns.
If you want to play skirmish (which I assume), you can be almost competitive with one core set.
Things you might want to get ...
If you want to play the Empire:
* 1-2 Additional Imperial Officers as singles
* 1-2 Additional Royal Guards as singles
* The Royal Guard Champion VIllain pack
If you want to play the Rebels:
* 2x Rebel Saboteurs
* 1x Rebel Troopers
With that you should be good and be able to build lists that have a decent chance of winning.
Most of the available expansions are unique figures, so definitely don't require multiple purchases.
And it's technically unnecessary to buy any of them if you're interested in the campaign rather than skirmish, as cardboard tokens are provided. But the figures are nice to have and include new cards and side missions etc.
Jacenat beat me to it and is correct in all respects.
The nice thing about this game is that you pretty much get everything you need from the core set to play with up to 5 people in the campaign and up to two in the skirmish games.
The ally and villain packs are completely optional. I would buy the game, play a campaign and some skirmishes to see if you like it, then buy the ally and villain packs if you and your friends find its something you enjoy.
We play both skirmish and and campaign, so I've found the ally/villain packs to be a great addition. Each one comes with extra campaign missions and/or an imperial Agenda deck, two skirmish scenarios, tons of command cards that make skirmish way more fun, an extra deployment card (including an Elite version if it exists), and finally a nifty little plastic model.
There's another thread where people discuss which packs give the most value:
https://community.fantasyflightgames.com/topic/180960-ally-and-villain-packs-ranking/
Don't forget an expansion pack is coming out soon. That will probably be have the highest cost to value ratio when it comes out if you want to expand.