TLDR - I will save you having to skip to the end. Just buy it .... Ignore the mountain of DLC and simply get the base game on sale and give it a go.

Never IMO has a game been more worth that initial effort of learning the ropes. I simply love this game and would recommend it to everyone. Explore the galaxy and expand your territory, design your ships and build fleets, befriend aliens and form alliances, join the galactic community or create Federations or better still conquer, subjugate, enslave or simply purge the Xeno scum, so many ways to play. When you start a game you can customise your species not just in how it looks but by spending points and picking Traits, Ethics and Civics that can be positive or negative and can make a significant impact on how your species plays. Like all games there is a meta build or you can just have FUN and create what ever your heart desires. I love creating my species then trying to RP it. What ever you decide be warned at first you will die .. a LOT.

If there is something the game does not have then I can almost guarantee there is a mod to add it, from Star Trek to Star Wars to 40k its all there somewhere lol.

Now to the great big turd in the room .. DLC .. I have nothing against this at all and IMO if a game is to be supported for years after launch and be expanded then yes the devs need to make money it is a business after all.Others wont agree but hey this is MY opinion. Problem now is you will look at this game and crap your knickers at the list of DLC thinking "WTF ? over £100 for everything ,, no way in hell " ...... Well take a great big chill pill and relax.... I played this game for a long time with no DLC and loved it so much that over time when there was a sale I picked up the DLC that at first offered something I liked the look of. Then over more time I picked the others up just to support the company so that now I own it all. A YouTuber called ASPEC does some excellent videos describing all the DLC and I would recommend watching him.
NB if you are lucky and have a friend with Stellaris then you can play a game he hosts and get access to any DLC he owns for free while in multiplayer.

The game has changed so much from release and at each stage that upset a lot of people as you can read in the negative reviews. Overall I have been fine with the changes and think the game is in the main better now than at launch and the devs keep improving performance and now not only adding DLC but more importantly they are also going back and improving previous DLC.