See I don’t have any qualms with CA’s DLC model for basically the reason you stated. They mostly add new factions and a few new units. You can absolutely go ahead and buy game 1+2 for what, 35 bucks on sale? I bet they’ve bundled them cheaper. And see if you like the game enough that you are bored with the mere 20 or so factions. Yes you’ll have less options in competitive multiplayer, but that is a very small percentage of players.
and @Chaplin figuring out the reason why you want to bring one unit over another is roughly 75% of the fun I have playing Total Wars. Sometimes units only real difference is economic. One is better and costs more. But most of the time the units have some small niche they are good at or terrible at. Campaign’s biggest flaw to me is that units that aren’t good in sieges have no place in your armies, which basically removes all skirmishers and chariots from consideration. These units are good in open field battles, but 50%+ of your battles are sieges in this game.