Sigh, right I give up - as a purchaser if 1.0 what is my vanilla game running at now and what do I need to make it awesome
Haha. Not to sound cheeky, but I honestly recommend grabbing all of them at some point (though as usual, I also recommend waiting for a sale). The 5 major expansions (Utopia, Apocalypse, MegaCorp, Federations, & Nemesis) and 4 story packs (Leviathans, Synthetic Dawn, Distant Stars, & Ancient Relics) all enhance the base game to a greater or lesser degree, via new features and/or content.
The 4 species packs are admittedly more a case of YMMV, however. (Humanoids are my personal favorite -- even though it doesn't currently include changes to gameplay -- largely for the additional advisor voices, which I found greatly improved my sense of immersion.) Barring one of them having an especially strong appeal to you, they can easily be skipped. It'll be interesting, though, to see how the species packs stack up against each other after the 3.1 "Lem" update this fall, as Lithoids & Necroids are getting spruced up, and Humanoids & Plantoids are (finally!) getting their own gameplay features added in.
All that being said, this is my current "tier list" for Stellaris DLC. It's slightly revised from what I posted a month or two ago. (Note: I do not own Necroids.)
S.) Utopia
A.) Distant Stars, Apocalypse, Leviathans
B.) Federations, Ancient Relics, Synthetic Dawn
C.) MegaCorp, Humanoids
D.) Nemesis, Lithoids
F.) Plantoids
At the very least, I would grab
Utopia and the DLC I've listed on A-tier. I highly recommend picking up the ones on B-tier as well, although I realize that'd be a lot of DLC to purchase all at once.