I don't have D4 or D2

But I do have Disgaea 3. The DLC isn't exactly needed. But I know if you want to know a little extra about Beryl there episodic dlc for her is worth it.

And if you're asking about this in terms of trophies, no DLC is needed to get 100% and the Plat. HOWEVER, the Prinny and Adell are good for making the trophy grind much easier to deal with.

I played Disgaea 3, and if you liked Disgaea or 2 you might like some of the character DLC.

That depends if you end up liking certain characters or the game in general. Hard to choose beforehand xD.

I've played disgaea 3 out of that list. I know it's not in the question but I'm currently playing disgaea 5 and it's my favourite, it's a bit more new player friendly but because it's got a stronger story that seems longer than the other ones.

Can't really say anything about the doc as I've never bought any for disgaea games.

well if you happen to have a vita, i think the all the dlc is included in Disgaea 3 vita and Disgaea 4 vita. I think games like Disgaea gel well with a vita. That might be just me though.

I can only vouch for disgaea 4 since that's the only one among the 3 i've played so far. For disgaea 4 you have an extra story dlc for fuuka and desco. If you happen to like them both, then you might enjoy it. On the other hand, Disgaea 4 vita has an extra story exclusive for it and also has the cheat shop from Disgaea D2.

I personally haven't played both of the dlc myself since I recently changed to the Japanese Vita version.