Like Mike said, the major improvements are in the free patches. There’s no need to pick up the DLC as a new player. If you really want to play around with Commonwealth nations or play in the Balkans and want more unique experiences for those countries, then pick up the first two DLCs. Until then, the base game would give you plenty to play with.
It’s not perfect by any stretch, but in my opinion HOI4 is an excellent WW2 grand strategy game. And has been mentioned previously in this thread, the production system is the bees knees.