The DLC's add a ton to the game. New storylines, new sectors (more space), new ships and buildings, new mechanics (in some), etc.

Do I think you need them all at the start? Probably not? I don't know how much things cost at this point since I've gotten all the DLC's when they came out since I got the game so it's hard for me to say whether it's "worth it" price wise. But just from a content and fun standpoint? They add a LOT to the game and are worth having for sure.

IMHO, the Split ships are the ones I prefer flying generally, so I would recommend that DLC if nothing else. But every faction has one or two ships that are best in class, it's balanced nicely that way. I just know that if I had to start the game from scratch without Split ships, I would be sad. :-D