Summer Sale, perhaps. (So many games.)
Yeah, you're best bet is to wait for a sale. If you're going to get Stellaris it's probably wise to get Utopia and Apocalypse with it. Megacorp supposedly isn't amazing and Synthetic Dawn is for machine empires. Leviathans and Distant Stars are more for flavor. The rest is purely cosmetic/vanity.
I picked up the base game, Utopia, Apocalypse and Leviathans recently. It's certainly fun but not without it's weaknesses. As an example, some stuff with population management can get irritating
. Late game micro can become a burden as well.
Also, keep in mind it's not like a lot of the popular strategy games. Total War games are more battle simulators with strategic maps, for instance. Civ is more 4x. Stellaris is arguably a bit of a hybrid between Civ and Grand Strategy games, like Crusader Kings, with more in common with the latter of the two.