No Storyline, might as well forget that.
Casual is fine, and about the same progression as past elite games until you hit a wall at around 5mil at which point significant cash is needed for upgrades in terms of ship equipment and the larger ships.
Is it worth a buy, Yes in sale
Forget about expansions for the time being.
No storyline? What about the Thargoids and the Formidine Rift mystery?
If you don't buy the Horizons expansion then you won't be able to explore planets and take part in the current storyline to a huge extent. I love landing on a planet and driving around in my SRV. You never know what you might find...