As of 2019, Crusader Kings 2 has a on Steam. You can play as a Christian feudal lord for as long as you want without spending a cent.

Unlocking all DLC gives you access to transformative mechanics, including: : Deepens religious gameplay and adds warrior lodges. The Old Gods

The best that work with the free base game Share public link

If you need help identifying which DLCs add the features you want (e.g., Republics, Nomads, Religions, Retinues, etc.), I can explain those or help you plan a cost-effective purchase strategy. Just let me know.