I plan on playing through the games in order. I do not have any prior knowledge of story or lore and learn as I go. I'm not going to make myself stick to a game if it's not 100%'d, sometimes I will move forward and go back to a game later. As of writing (3/4/25), The only achievement I have left for the first game, Kingdom Hearts (KH1), is the proud mode acheivement. I've completed Kingdom Hearts: Re:Chain of Memories (KH ReCoM) to catch up with the story completely and move on to KH2. You need to go back after Kingdom Hearts 358/2 Days ANYWAYS to 100% ReCoM which is after KH2... So right now is KH2! Yay!!
You can buy the KH games here on Steam for PC. You can also buy them through the Epic Games store or as a bundle for PS3 and PS4.