This is the full version! The full game with step 1-3 is completely free, with step 4 (kinda like one big epilogue which will also be free!) still in the making. It is optional to buy step 1 and step 2 dlcs (as well as other future dlcs), but you won't miss out anything from the main plot even if you choose not to!
I ended buying them anyway because it is just THAT good, but you don't necessarily have to and will have just as much fun! I hope this helps :))