There's only really one DLC at this point, which is horizons, and if you buy the game now I'd suggest to get the whole package. But basically the game keeps being extended, and the DLC is more like season passes, in that you get about a years worth of content updates with each one.
Even if you don't buy the new season pass (it's not quite clear yet when the next one will come up, because the current one is a bit behind schedule) you usually still get a lot of the updates like except when they are related to the core features of the season pass (for example, I still don't own horizons, but I still got all the new ships, the NPC avatars and some other stuff).
Other than that, all DLC is purely cosmetic (ship skins, bobbleheads, that kind of thing).