Yeah, it's an unusual game in that regard.
Rather than being sequels of each other, 1+2+3 build on each other and you can use all of the DLCs on one massive map.
Personally, I enjoyed WH2, and ended up buying WH1 so that I could stack them together and use the massive map. But at the same time I found it was getting increasingly impenetrable for a casual player (I would constantly get surprised by enemy lords that I had no knowledge of, and looking online the long-time players knew what each lord, spell, magic, troops would bring etc and how to counter them) and the sheer quantity of DLC involved was monstrous. For what it's worth, I just had a look and I've bought 2 DLCs for WH2. To buy the remaining WH1, 2 and 3 DLCs would cost £257.
I thought it might be fun to play the Skaven, and saw a couple of online guides, but then discovered that unless I bought some DLC I'd actually be locked out of all of the fun units. I then fancied trying out the Tomb Kings, but I think that was impossible without buying DLC.
So I stopped playing a while ago, and after WH3 got bad feedback initially I decided against getting back into it.
It is certainly a fun game, but I think it's really difficult to just dip your toe into.
And I guess I always have the nagging feeling that it has hurt the historical releases, but that's going wildly off topic.