I've never used any of the paid DLC stuff. I might at some point, but it's certainly not required by any means.

Lots of totally free mods, though.

Edit to add: The paid DLCs add new mechanics to the base game and are basically the same as old school expansion packs were for offline games. It's not like that lootbox bullshit or anything scammy. Also, these DLCs are ongoing development. Game's been out since like 2015 and they just release a new DLC last month.

Also, I'll add that I am not affiliated with Colossal Order (developer) or Paradox Interactive (publisher) and this is not a paid advertisement. I just really like the game, just not enough to pay for DLC.

A bunch of the DLC are just radio and custom building little DLC. I think there are a total of 12 legitimate expansions, but none of them are necessary to enjoy the game and it has thousands of quality mods available for free.

They go on sale all the time. I got the base game plus all expansions at the time for like $50