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:

CK3 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.

I'd strongly advise to start with no DLC, for either version. These are two games that are 1) huge and complex 2) very difficult. You will take a while to learn the ropes, and you won't miss stuff to pay attention to during that time (in fact it will be overwhelming regardless of DLC, that is just the nature of the game - and part of the appeal). You may also end up deciding this isn't for you at all, or that you like it but not enough to invest more moneys into DLC.

The base games are perfectly good on their own, and in fact have an amazing cost benefit (CK2 is free so it is hard to beat on that front, but any paradox game). The DLC have just a lot less stuff for the price, vastly diminishing returns, and the discounts on sale are worse... but it is fine, because for people that play hundreds of hours improving the game even a bit is still an awful lot of value. If you don't know whether you'll play hundreds of hours, getting the DLC to have "the best possible version" might be a very bad deal. CK2 with all DLC is better but not that much better. If anything, the base game is underpriced, because only people that buy the base game can start buying the more profitable DLC.

About CK2 vs CK3, the newer one is much much easier to get started with. It is likely a better game, though I have played a lot less of it to compare (and it will still improve over time). So it is the best recommendation. But CK2 without DLC is free, which is the perfect price to give it a try, learn how to play, because why not. If you don't enjoy it then try CK3 which is more approachable, if you like it you can think about the subscription (likely better value unless you keep paying for it for years) or a large bundle on sale... or still try CK3 and see which you like better.

If you still want to buy just a few pieces of CK2 DLC, I guess I can dig up my old list. For CK3, go for the older ones that are already at 50% off, decide if you want cosmetics (not the most popular xD), play as something completely different, or general improvements. DoctorJunglist's discussion breaks it down nicely.

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.

Well if you already play CK2 and like it, I think my first recommendations would be Holy Fury (last DLC, quite packed with good general improvements), Way of Life and Conclave (general improvements I quite like), and Pagan Fury (earlier start date and pagans are quite fun), and Reaper's Due is also general stuff but not as good. Monk and Mystics has secret societies, it is a bit wacky, fun for a few playthroughs but not something you'll always use.

Then we get the culture specific stuff: The Republic is a unique play style, fun but small. Horse Lords and Jade Dragon work well together, it is cool for playing in the region, very different mechanics but the weirdness make it less replayable IMO. Sword of Islam is a bit generic and bland, but there are so many Muslim characters it makes some impact. Rajas of India is kind of the midpoint between Sword of Islam and Jade Dragon. Oh, and Charlemagne, it gives you another starting date (even earlier) but mostly that is it, playing the Charlemagne story is meh and it is too early, it isn't as good.

Then the small event stuff: Legacy of Rome and Sons of Abraham hardly matter, but enable a few events and religions to play as. I'd get it as part of a bundle only. And Sunset Invasion, well, you might leave it on once for the novelty but I'd rather disable the Aztec invasion (historicity aside, it isn't particularly fun).

The Royal Collection has all the gameplay stuff, the rest is cosmetic. On sale should be the cheapest bet. Though I'll say, I do enjoy some cosmetics: the different portraits for the various regions, and the music pack is really good. The rest I couldn't even notice, so I don't have it.

Well if you already play CK2 and like it, I think my first recommendations would be Holy Fury (last DLC, quite packed with good general improvements), Way of Life and Conclave (general improvements I quite like), and Pagan Fury (earlier start date and pagans are quite fun), and Reaper's Due is also general stuff but not as good. Monk and Mystics has secret societies, it is a bit wacky, fun for a few playthroughs but not something you'll always use.

Pagan Fury is just a music pack, you probably meant "The Old Gods" DLC. :).

A very good list, I will agree with most of it - except I would describe Reaper's Due a little bit more favorable and I would place Charlemagne much higher - I always liked longer campaigns and on top of that DLC also lets you create custom (ahistorical) kingdoms and viceroys vassals. Steam also mentions regency overhaul, which is very important, but I think it's part of a free patch actually.

Well if you already play CK2 and like it, I think my first recommendations would be Holy Fury (last DLC, quite packed with good general improvements), Way of Life and Conclave (general improvements I quite like), and Pagan Fury (earlier start date and pagans are quite fun), and Reaper's Due is also general stuff but not as good. Monk and Mystics has secret societies, it is a bit wacky, fun for a few playthroughs but not something you'll always use.

Pagan Fury is just a music pack, you probably meant "The Old Gods" DLC. :).

A very good list, I will agree with most of it - except I would describe Reaper's Due a little bit more favorable and I would place Charlemagne much higher - I always liked longer campaigns and on top of that DLC also lets you create custom (ahistorical) kingdoms and viceroys vassals. Steam also mentions regency overhaul, which is very important, but I think it's part of a free patch actually.

Oops yeah I confused the two. To be fair it has furious pagans. xD