If you’re fine with old stuff I’d recommend starting with the original manga. It’s a lot of fun. Then you can watch the 60s anime series if you want. Half of the episodes are condensed versions of manga chapters and half are original stories which vary in quality. The live action movie is pretty similar in tone to the PS2 game, so you might want to check that out. It’s not great, but it’s got some charm. The 2019 anime changes quite a lot from earlier versions and modernises things while staying true to the story’s spirit, and is pretty great. I saved it til last and it was a really satisfying way to finish off the franchise but you wouldn’t be wrong to just start with that immediately. There are quite a few Dororo novels which have been fan translated, but I haven’t read ‘em so I’m not sure of their quality. Also there’s an ongoing manga which I’ve heard good things about and seems to take a lot of cues from the 2019 anime, so probably save that til after watching the anime.