I've bought base Stellaris over 2 years ago and have not played it yet.
Finally I want to start playing this game, but I would like to buy 3-4 add-on's. Which 4 add-on's would you reccomend? (having in mind that for 99.99% I will not buy any more of them)
Cheers.

There is a good overview:
https://interreviewed.com/the-best-stellaris-dlc-a-complete-guide/

I would say, the "big" DLCs are worth a shot, depending on your preferences.
Also, all the story packs are nice to have.
Then there are the species packs - choose any you might like. They all add something. Have a look at the ship designs also, some of them look awsome, othes are average.

Also: Wait for sales!

Wow, this link looks like exactly what I need. Thanks!

I bought Stellaris a couple months ago and last month they were doing a huge sale and they offer 3 packets of DLC, the species pack, the expansion pack, and the story pack. I got all of them together for all the DLC for around $50. I don't know if GOG is still offering that deal.

Utopia is the best DLC for Stellaris, and I would go so far as to say its features are core and integral to the Stellaris experience. One of the key changes it brought are the three different ascension paths (Psionic, Biological, and Synthetic) that change a lot of your empire's playstyle and means empires that start out very similarly can develop in radically different directions, Megastructures are important options to give you something to build with your superpower late-game economy, and there are countless minor features like different slavery policies so slavery is actually flexible enough to use (in the base game you only get chattel slavery, which is only good early-game and basically forces you to abolish slavery in the late-game since it hurts more than it helps).

The other DLC's have different pros and cons, and I would recommend looking at the feature-set and deciding what looks good to you. The only one I'd recommend avoiding right now is Nemesis, which is overpriced and has a feature list comparable to DLC's half its cost. Even Paradox has admitted this, and I saw a developer comment that they plan on retroactively adding more content at a later date.

https://www.youtube.com/watch?v=3cCkkT39l_Y
found this youtube guide to dlc content to be help
goes more in depth of what you are getting for them

Over the course of the last few years, I've gotten them all*, on sale. I didn't get the species packs.

Federations, Ancient Relics, Federations and Utopia I think are the more interesting ones to me.

put them all on your wishlist, they go on sale often.

* up until the latest one.