Best Expansion to Get

@WildestGetaway  I can give you my personal opinion, but there aren't a lot of other players who post here; most people only come by when they need help with their games.  You might have better luck asking on the official Sims forums.  You'd need to be a full member to start your own thread, but I could start one for you if you wanted.  Or you could post in an existing thread, as long as it was on topic.

https://forums.thesims.com/en_US/categories/general-discussion

As for playability, Late Night itself is very mild, especially compared to Seasons.  The problem is with Bridgeport, the world that comes with LN.  There are numerous routing issues, plus sims can get stuck in elevators or in the darkened out parts of apartment buildings.  This is all fixable though—a player named Ellacharmed uploaded a fixed version of Bridgeport, and you can edit buildings yourself if you choose.  Or you could play in a different world and add the LN features you want.

With Supernatural, you can turn off the occult types you don't like.  But if you're not interested in any of them, you might not get a whole lot out of the pack.  For what it's worth, Supernatural is more demanding than Late Night (aside from Bridgeport's issues), although still not as bad as Seasons.

For stuff packs, my favorite is Master Suite.  But none of the stuff packs add any gameplay; they're just, well, stuff.  You can see all items added with each one on Carl's Sims 3 site.  Each expansion and stuff pack has its own page, linked here:

https://www.carls-sims-3-guide.com/expansionpacks.php

Which store world you'd like best will probably depend on your personal taste.  The only one that's ever on sale is Sunlit Tides, at something like 1150 points for the standard version.  (The venue that comes with the gold version is also available separately, and on sale for 500 or so.)  You might also look into player made worlds.  Some of them are gorgeous, and many only use stuff from the base game or maybe a couple of packs.  Rflong has a great reputation for well-made worlds that are lots of fun to play.

P.S.  If you want to talk about your computer's performance, just let me know.  It would be helpful to see a dxdiag, which would cover a lot of ground all at once.

https://help.ea.com/en-us/help/pc/how-to-gather-dxdiag-information/