Ex fan there. If you go for CK 2, don't buy any DLC, the game is ftp and there is a subscription with all DLC for 5€/month.
Every DLC is 300€. Any amount of DLC between 0 and all is sub-optimal as game is not tested for those configuration and you probably wont play 5 years straight.

I don't have played at CK 3 yet so no info on it.

I have all DLC's for CKII and probably all of them for CKIII but for a new player I agree with amatai - it's much easier to buy a DLC subscription for a month or a few and see how much you will like the game and than decide on how much you want to spend on it. How much you are willing to buy is an important question before any recommendations.

Another advice - live on subscription till the Steam sale starts. Buying them will be expensive even on sale, and even more - without it.

If you want some advice on CK3 DLCs, I found a nice reddit comment giving some advice on that matter:

QuoteCK3 DLC can be broken into 3 categories. Cosmetic DLCs (Garments of the holy roman empire, north africa attire pack, etc) You can skip without worrying what you're missing.

Regional DLC packs (Legacy of persia, fate of iberia, northern lords, etc) only affect specific areas. If you want to play as a viking then Northern Lords is a must-have, but if you're forming a kingdom in Italy it's not gonna do much for you.

The last kind of packs are the Game Sytem packs. These usually include 1 free game system for everyone and 1 game system locked behind the DLC. For Tours and Tournaments the wonderful Travel system came to everyone, but if you want to attend/hold tournaments/weddings/tours then you need the DLC. Legends of the dead (which I think has the worst reviews) is ironic, because the free update, plagues, was much better than the paid mechanic of legends and funerals. Roads to Power is unique in that there are two systems locked behind the DLC, the Byzantine administration stuff and landless play. It also has a higher price tag but I think its a bigger than usual expansion.

For me personally, I would recommend Tours and Tournaments and Roads to Power as the big 2 DLCs. Then I would pick up Wards & Wardens and Friends & Foes just because they're cheaper. Then I would pick up The Royal Court, it's not the best DLC but it does some unique stuff. Lastly I would look at Legends of the Dead when it starts going on sale 50% off.

I started playing base CKII again. Forgot how to mingle, getting to king from count is harder than i remember. Killing all the male offspring is hard work, not to mention fails when the only surviving heir is not in a matrilineal marriage.

I really liked the formula of CKII, that's why i consider buying DLCs. (I have stellaris but that is so different, i can't really play it too long.) Thanks for the breakdown, I'll probably wait until Paradox sells a bundle or knocks a few bucks off the price of the base game.